feat(ssh): wave 2 — route filesystem/shell tools behind ExecBackend

Wave 2 of transparent SSH support (4 parallel owner-agents on disjoint
tool packages). The tools now resolve an ExecBackend per-call from
ctx.computerId and call backend.spawn / backend.readFile / etc. instead of
node:fs and node:child_process directly — so they are transport-agnostic
(local now; remote over SSH later, transparent to the agent). Still LOCAL-ONLY
this wave (computerId always undefined -> LocalExecBackend, behavior-identical).

- tool-shell: factory takes resolveBackend; execute calls backend.spawn.
  spawn.ts DELETED (realSpawn was a verbatim duplicate of exec-backend's
  LocalExecBackend.spawn — logic moved to the sanctioned shared package).
  manifest dependsOn:[exec-backend]; host.getService at activation.
- tool-read-file: readFile/stat/readdir -> backend.* (pure logic untouched;
  ENOENT .code branches kept).
- tool-write-file: exists/stat/writeFile -> backend.* (pure logic untouched).
- tool-edit-file: readFile/writeFile -> backend.* + forward-compatible REMOTE
  diagnostics skip (ctx.computerId set -> skip LSP, return empty — plan §6.1;
  local path byte-identical to today). LSP lookup stays lazy.
- orchestrator: pre-wired @dispatch/exec-backend dep into the 4 tool
  package.jsons + bun install (build/config, my lane) so isolated verify
  resolved cleanly; agents added the ../exec-backend tsconfig ref.

Verified: tsc -b EXIT 0, biome clean, 1599 vitest pass (was 1592).
Refs: notes/ssh-support-plan.md (decisions §0.5/§13). No merge or push.

3 files changed, 141 insertions, 32 deletions

diff --git a/packages/tool-read-file/src/extension.ts b/packages/tool-read-file/src/extension.ts
index 8c3a064..5a0b7c5 100644
--- a/packages/tool-read-file/src/extension.ts
+++ b/packages/tool-read-file/src/extension.ts
@@ -1,3 +1,4 @@
+import { execBackendHandle } from "@dispatch/exec-backend";
 import type { Extension } from "@dispatch/kernel";
 import { createReadFileTool } from "./read-file.js";
 
@@ -11,8 +12,11 @@ export const extension: Extension = {
 		activation: "eager",
 		capabilities: { fs: true },
 		contributes: { tools: ["read_file"] },
+		// Host activates exec-backend first → host.getService at activation is safe.
+		dependsOn: ["exec-backend"],
 	},
 	activate(host) {
-		host.defineTool(createReadFileTool(process.cwd()));
+		const resolveBackend = host.getService(execBackendHandle);
+		host.defineTool(createReadFileTool({ resolveBackend, workdir: process.cwd() }));
 	},
 };
diff --git a/packages/tool-read-file/src/read-file.test.ts b/packages/tool-read-file/src/read-file.test.ts
index 619ba34..bac5902 100644
--- a/packages/tool-read-file/src/read-file.test.ts
+++ b/packages/tool-read-file/src/read-file.test.ts
@@ -1,6 +1,7 @@
 import { mkdir, mkdtemp, rm, writeFile } from "node:fs/promises";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
+import { localExecBackend } from "@dispatch/exec-backend";
 import { createLogger, type ToolExecuteContext } from "@dispatch/kernel";
 import { afterEach, beforeEach, describe, expect, it } from "vitest";
 import {
@@ -25,6 +26,15 @@ function stubCtx(overrides?: Partial<ToolExecuteContext>): ToolExecuteContext {
 	};
 }
 
+/**
+ * Build a read_file tool wired to the real local ExecBackend (node:fs,
+ * behavior-identical to today's inline calls). No `@dispatch/*` mocking — the
+ * real fs edge is exercised, matching the constitution's strict-core rule.
+ */
+function makeTool(workdir: string) {
+	return createReadFileTool({ resolveBackend: () => localExecBackend, workdir });
+}
+
 let workdir: string;
 
 beforeEach(async () => {
@@ -151,7 +161,7 @@ describe("createReadFileTool", () => {
 		const filePath = join(workdir, "hello.txt");
 		await writeFile(filePath, "hello\nworld\n", "utf8");
 
-		const tool = createReadFileTool(workdir);
+		const tool = makeTool(workdir);
 		const result = await tool.execute({ path: "hello.txt" }, stubCtx());
 
 		expect(result.isError).toBeUndefined();
@@ -163,7 +173,7 @@ describe("createReadFileTool", () => {
 		const filePath = join(workdir, "lines.txt");
 		await writeFile(filePath, "a\nb\nc\nd\ne\n", "utf8");
 
-		const tool = createReadFileTool(workdir);
+		const tool = makeTool(workdir);
 		const result = await tool.execute({ path: "lines.txt", offset: 2, limit: 2 }, stubCtx());
 
 		expect(result.isError).toBeUndefined();
@@ -171,7 +181,7 @@ describe("createReadFileTool", () => {
 	});
 
 	it("returns error for missing file", async () => {
-		const tool = createReadFileTool(workdir);
+		const tool = makeTool(workdir);
 		const result = await tool.execute({ path: "nonexistent.txt" }, stubCtx());
 
 		expect(result.isError).toBe(true);
@@ -182,7 +192,7 @@ describe("createReadFileTool", () => {
 		const filePath = join(workdir, "empty.txt");
 		await writeFile(filePath, "", "utf8");
 
-		const tool = createReadFileTool(workdir);
+		const tool = makeTool(workdir);
 		const result = await tool.execute({ path: "empty.txt" }, stubCtx());
 
 		expect(result.isError).toBeUndefined();
@@ -194,7 +204,7 @@ describe("createReadFileTool", () => {
 		const filePath = join(workdir, "short.txt");
 		await writeFile(filePath, "one\n", "utf8");
 
-		const tool = createReadFileTool(workdir);
+		const tool = makeTool(workdir);
 		const result = await tool.execute({ path: "short.txt", offset: 100 }, stubCtx());
 
 		expect(result.isError).toBe(true);
@@ -202,7 +212,7 @@ describe("createReadFileTool", () => {
 	});
 
 	it("never throws on bad input (always returns ToolResult)", async () => {
-		const tool = createReadFileTool(workdir);
+		const tool = makeTool(workdir);
 
 		const inputs = [null, undefined, 42, "string", {}, { path: "" }, { path: 123 }];
 		for (const input of inputs) {
@@ -213,12 +223,12 @@ describe("createReadFileTool", () => {
 	});
 
 	it("concurrencySafe is true", () => {
-		const tool = createReadFileTool(workdir);
+		const tool = makeTool(workdir);
 		expect(tool.concurrencySafe).toBe(true);
 	});
 
 	it("has correct name and parameters shape", () => {
-		const tool = createReadFileTool(workdir);
+		const tool = makeTool(workdir);
 		expect(tool.name).toBe("read_file");
 		expect(tool.parameters.type).toBe("object");
 		expect(tool.parameters.required).toEqual(["path"]);
@@ -231,7 +241,7 @@ describe("createReadFileTool", () => {
 			const filePath = join(ctxDir, "ctx-file.txt");
 			await writeFile(filePath, "from ctx cwd", "utf8");
 
-			const tool = createReadFileTool(workdir); // baked workdir is different
+			const tool = makeTool(workdir); // baked workdir is different
 			const result = await tool.execute({ path: "ctx-file.txt" }, stubCtx({ cwd: ctxDir }));
 
 			expect(result.isError).toBeUndefined();
@@ -245,7 +255,7 @@ describe("createReadFileTool", () => {
 		const filePath = join(workdir, "baked-file.txt");
 		await writeFile(filePath, "from baked workdir", "utf8");
 
-		const tool = createReadFileTool(workdir);
+		const tool = makeTool(workdir);
 		const ctx = stubCtx();
 		// Ensure cwd is undefined
 		expect(ctx.cwd).toBeUndefined();
@@ -260,7 +270,7 @@ describe("createReadFileTool", () => {
 		await writeFile(join(workdir, "zebra.txt"), "z", "utf8");
 		await writeFile(join(workdir, "alpha.txt"), "a", "utf8");
 
-		const tool = createReadFileTool(workdir);
+		const tool = makeTool(workdir);
 		const result = await tool.execute({ path: "." }, stubCtx());
 
 		expect(result.isError).toBeUndefined();
@@ -270,7 +280,7 @@ describe("createReadFileTool", () => {
 	it("returns empty-directory message for an empty dir", async () => {
 		await mkdir(join(workdir, "empty-dir"));
 
-		const tool = createReadFileTool(workdir);
+		const tool = makeTool(workdir);
 		const result = await tool.execute({ path: "empty-dir" }, stubCtx());
 
 		expect(result.isError).toBeUndefined();
@@ -280,7 +290,7 @@ describe("createReadFileTool", () => {
 	it("reads a file unchanged (regression: line numbers + offset/limit)", async () => {
 		await writeFile(join(workdir, "regression.txt"), "a\nb\nc\nd\ne\n", "utf8");
 
-		const tool = createReadFileTool(workdir);
+		const tool = makeTool(workdir);
 		const result = await tool.execute({ path: "regression.txt", offset: 2, limit: 3 }, stubCtx());
 
 		expect(result.isError).toBeUndefined();
@@ -288,10 +298,89 @@ describe("createReadFileTool", () => {
 	});
 
 	it("returns not-found for a nonexistent path", async () => {
-		const tool = createReadFileTool(workdir);
+		const tool = makeTool(workdir);
 		const result = await tool.execute({ path: "nonexistent-path" }, stubCtx());
 
 		expect(result.isError).toBe(true);
 		expect(result.content).toContain("not found");
 	});
+
+	it("routes fs calls through resolveBackend(ctx.computerId) (transport seam)", async () => {
+		// A fake backend records what it is asked to do. Proves the tool programs
+		// against the ExecBackend surface (not node:fs) and that the resolver is
+		// invoked with ctx.computerId — the SSH seam. No real fs involved.
+		let statCalls = 0;
+		let readFileCalls = 0;
+		let readdirCalls = 0;
+		let receivedComputerId: string | undefined = "__sentinel__";
+		const fakeBackend = {
+			spawn: async () => ({ exitCode: 0, timedOut: false, aborted: false }),
+			readFile: async (path: string) => {
+				readFileCalls++;
+				expect(path).toContain("seam.txt");
+				return "fake-line-1\nfake-line-2";
+			},
+			writeFile: async () => {},
+			stat: async (path: string) => {
+				statCalls++;
+				expect(path).toContain("seam.txt");
+				return { isFile: true, isDirectory: false };
+			},
+			readdir: async () => {
+				readdirCalls++;
+				return [];
+			},
+			exists: async () => true,
+		} as const;
+
+		const tool = createReadFileTool({
+			resolveBackend: (computerId) => {
+				receivedComputerId = computerId;
+				return fakeBackend;
+			},
+			workdir,
+		});
+		const result = await tool.execute({ path: "seam.txt" }, stubCtx({ computerId: "prod-ssh" }));
+
+		expect(receivedComputerId).toBe("prod-ssh");
+		expect(statCalls).toBe(1);
+		expect(readFileCalls).toBe(1);
+		expect(readdirCalls).toBe(0);
+		expect(result.isError).toBeUndefined();
+		expect(result.content).toBe("1: fake-line-1\n2: fake-line-2");
+	});
+
+	it("resolves the local backend when ctx.computerId is undefined (backward compat)", async () => {
+		// computerId undefined → resolver returns localExecBackend → real fs.
+		const tool = createReadFileTool({ resolveBackend: () => localExecBackend, workdir });
+		const filePath = join(workdir, "compat.txt");
+		await writeFile(filePath, "real fs via backend\n", "utf8");
+
+		const result = await tool.execute({ path: "compat.txt" }, stubCtx());
+
+		expect(result.isError).toBeUndefined();
+		expect(result.content).toContain("1: real fs via backend");
+	});
+
+	it("preserves ENOENT .code branch through the backend (fake backend throws)", async () => {
+		const enoent = Object.assign(new Error("ENOENT: no such file or directory"), {
+			code: "ENOENT",
+		});
+		const fakeBackend = {
+			spawn: async () => ({ exitCode: 0, timedOut: false, aborted: false }),
+			readFile: async () => "unused",
+			writeFile: async () => {},
+			stat: async () => {
+				throw enoent;
+			},
+			readdir: async () => [],
+			exists: async () => false,
+		} as const;
+
+		const tool = createReadFileTool({ resolveBackend: () => fakeBackend, workdir });
+		const result = await tool.execute({ path: "ghost.txt" }, stubCtx());
+
+		expect(result.isError).toBe(true);
+		expect(result.content).toBe('Error: File "ghost.txt" not found.');
+	});
 });
diff --git a/packages/tool-read-file/src/read-file.ts b/packages/tool-read-file/src/read-file.ts
index 216f165..b88c241 100644
--- a/packages/tool-read-file/src/read-file.ts
+++ b/packages/tool-read-file/src/read-file.ts
@@ -1,5 +1,5 @@
-import { readdir, readFile, stat } from "node:fs/promises";
 import { resolve } from "node:path";
+import type { ExecBackend, ExecBackendResolver, StatResult } from "@dispatch/exec-backend";
 import type { ToolContract, ToolResult } from "@dispatch/kernel";
 
 const DEFAULT_LIMIT = 500;
@@ -83,11 +83,21 @@ export function formatDirectoryEntries(entries: readonly DirEntry[], dirPath: st
 }
 
 /**
- * Factory: create a read_file ToolContract bound to a working directory.
- * The working directory is injected so the tool is testable.
+ * Factory: create a read_file ToolContract.
+ *
+ * `resolveBackend` is the injected seam: each `execute` resolves an
+ * `ExecBackend` from `ctx.computerId` (undefined → local `node:fs`; a set
+ * id → a remote SSH backend in a later wave). The tool programs against the
+ * `ExecBackend` surface, never `node:fs` directly, so it is transport-agnostic.
+ *
+ * `workdir` is the fallback base directory when `ctx.cwd` is omitted. It is
+ * injected so the tool is testable; `execute` prefers `ctx.cwd` when present.
  */
-export function createReadFileTool(workingDirectory: string): ToolContract {
-	const workdir = resolve(workingDirectory);
+export function createReadFileTool(deps: {
+	readonly resolveBackend: ExecBackendResolver;
+	readonly workdir?: string;
+}): ToolContract {
+	const workdir = deps.workdir !== undefined ? resolve(deps.workdir) : undefined;
 
 	return {
 		name: "read_file",
@@ -126,12 +136,21 @@ export function createReadFileTool(workingDirectory: string): ToolContract {
 			const { path: relPath, offset, limit } = validated;
 
 			const effectiveBase = ctx.cwd ? resolve(ctx.cwd) : workdir;
+			if (effectiveBase === undefined) {
+				return {
+					content:
+						"Error: No working directory (neither ctx.cwd nor a baked workdir was provided).",
+					isError: true,
+				};
+			}
 			const resolvedPath = resolve(effectiveBase, relPath);
 
+			const backend: ExecBackend = deps.resolveBackend(ctx.computerId);
+
 			// Stat to determine if this is a file or directory.
-			let pathStat: import("node:fs").Stats;
+			let pathStat: StatResult;
 			try {
-				pathStat = await stat(resolvedPath);
+				pathStat = await backend.stat(resolvedPath);
 			} catch (err: unknown) {
 				const code = (err as NodeJS.ErrnoException).code;
 				if (code === "ENOENT") {
@@ -143,28 +162,25 @@ export function createReadFileTool(workingDirectory: string): ToolContract {
 				};
 			}
 
-			// Directory listing branch.
-			if (pathStat.isDirectory()) {
-				let rawEntries: import("node:fs").Dirent<string>[];
+			// Directory listing branch. backend.readdir already returns
+			// {name, isDirectory}[] entries, so no per-entry collapse is needed.
+			if (pathStat.isDirectory) {
+				let entries: readonly DirEntry[];
 				try {
-					rawEntries = await readdir(resolvedPath, { encoding: "utf8", withFileTypes: true });
+					entries = await backend.readdir(resolvedPath);
 				} catch (err: unknown) {
 					return {
 						content: `Error reading directory: ${err instanceof Error ? err.message : String(err)}`,
 						isError: true,
 					};
 				}
-				const dirEntries = rawEntries.map((e) => ({
-					name: e.name,
-					isDirectory: e.isDirectory(),
-				}));
-				return { content: formatDirectoryEntries(dirEntries, relPath) };
+				return { content: formatDirectoryEntries(entries, relPath) };
 			}
 
 			// File branch — read the file.
 			let content: string;
 			try {
-				content = await readFile(resolvedPath, "utf8");
+				content = await backend.readFile(resolvedPath);
 			} catch (err: unknown) {
 				const code = (err as NodeJS.ErrnoException).code;
 				if (code === "ENOENT") {