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.

1 files changed, 21 insertions, 11 deletions

diff --git a/packages/tool-write-file/src/write-file.test.ts b/packages/tool-write-file/src/write-file.test.ts
index 6b316bc..d157eb2 100644
--- a/packages/tool-write-file/src/write-file.test.ts
+++ b/packages/tool-write-file/src/write-file.test.ts
@@ -1,6 +1,7 @@
 import { mkdir, mkdtemp, readFile, 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 { createWriteFileTool, decideOverwrite, validateArgs } from "./write-file.js";
@@ -19,6 +20,15 @@ function stubCtx(overrides?: Partial<ToolExecuteContext>): ToolExecuteContext {
 	};
 }
 
+/**
+ * Build a write_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 createWriteFileTool({ resolveBackend: () => localExecBackend, workdir });
+}
+
 let workdir: string;
 
 beforeEach(async () => {
@@ -116,7 +126,7 @@ describe("validateArgs", () => {
 
 describe("createWriteFileTool", () => {
 	it("creates a new file when overwrite is unset and the file is absent", async () => {
-		const tool = createWriteFileTool(workdir);
+		const tool = makeTool(workdir);
 		const result = await tool.execute({ path: "new-file.txt", content: "hello world" }, stubCtx());
 
 		expect(result.isError).toBeUndefined();
@@ -128,7 +138,7 @@ describe("createWriteFileTool", () => {
 	it("errors when the file exists and overwrite is unset", async () => {
 		await writeFile(join(workdir, "existing.txt"), "old content", "utf8");
 
-		const tool = createWriteFileTool(workdir);
+		const tool = makeTool(workdir);
 		const result = await tool.execute({ path: "existing.txt", content: "new content" }, stubCtx());
 
 		expect(result.isError).toBe(true);
@@ -141,7 +151,7 @@ describe("createWriteFileTool", () => {
 	it("overwrites an existing file when overwrite is true", async () => {
 		await writeFile(join(workdir, "existing.txt"), "old content", "utf8");
 
-		const tool = createWriteFileTool(workdir);
+		const tool = makeTool(workdir);
 		const result = await tool.execute(
 			{ path: "existing.txt", content: "new content", overwrite: true },
 			stubCtx(),
@@ -154,7 +164,7 @@ describe("createWriteFileTool", () => {
 	});
 
 	it("errors when overwrite is true but the file is absent", async () => {
-		const tool = createWriteFileTool(workdir);
+		const tool = makeTool(workdir);
 		const result = await tool.execute(
 			{ path: "nonexistent.txt", content: "data", overwrite: true },
 			stubCtx(),
@@ -165,7 +175,7 @@ describe("createWriteFileTool", () => {
 	});
 
 	it("errors when the parent directory does not exist", async () => {
-		const tool = createWriteFileTool(workdir);
+		const tool = makeTool(workdir);
 		const result = await tool.execute({ path: "no/such/dir/file.txt", content: "data" }, stubCtx());
 
 		expect(result.isError).toBe(true);
@@ -173,12 +183,12 @@ describe("createWriteFileTool", () => {
 	});
 
 	it("concurrencySafe is false", () => {
-		const tool = createWriteFileTool(workdir);
+		const tool = makeTool(workdir);
 		expect(tool.concurrencySafe).toBe(false);
 	});
 
 	it("has correct name and parameters shape", () => {
-		const tool = createWriteFileTool(workdir);
+		const tool = makeTool(workdir);
 		expect(tool.name).toBe("write_file");
 		expect(tool.parameters.type).toBe("object");
 		expect(tool.parameters.required).toEqual(["path", "content"]);
@@ -188,7 +198,7 @@ describe("createWriteFileTool", () => {
 	});
 
 	it("never throws on bad input (always returns ToolResult)", async () => {
-		const tool = createWriteFileTool(workdir);
+		const tool = makeTool(workdir);
 		const inputs = [null, undefined, 42, "string", {}, { path: "" }, { path: 123 }];
 		for (const input of inputs) {
 			const result = await tool.execute(input, stubCtx());
@@ -200,7 +210,7 @@ describe("createWriteFileTool", () => {
 	it("respects ctx.cwd over baked workdir", async () => {
 		const ctxDir = await mkdtemp(join(tmpdir(), "ctx-cwd-test-"));
 		try {
-			const tool = createWriteFileTool(workdir);
+			const tool = makeTool(workdir);
 			const result = await tool.execute(
 				{ path: "ctx-file.txt", content: "from ctx" },
 				stubCtx({ cwd: ctxDir }),
@@ -215,7 +225,7 @@ describe("createWriteFileTool", () => {
 	});
 
 	it("writes empty content", async () => {
-		const tool = createWriteFileTool(workdir);
+		const tool = makeTool(workdir);
 		const result = await tool.execute({ path: "empty.txt", content: "" }, stubCtx());
 
 		expect(result.isError).toBeUndefined();
@@ -225,7 +235,7 @@ describe("createWriteFileTool", () => {
 
 	it("writes content in subdirectory that exists", async () => {
 		await mkdir(join(workdir, "sub"));
-		const tool = createWriteFileTool(workdir);
+		const tool = makeTool(workdir);
 		const result = await tool.execute({ path: "sub/file.txt", content: "nested" }, stubCtx());
 
 		expect(result.isError).toBeUndefined();