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, 38 insertions, 24 deletions

diff --git a/packages/tool-write-file/src/write-file.ts b/packages/tool-write-file/src/write-file.ts
index 1317ce8..cf761b6 100644
--- a/packages/tool-write-file/src/write-file.ts
+++ b/packages/tool-write-file/src/write-file.ts
@@ -1,5 +1,5 @@
-import { access, stat, writeFile } from "node:fs/promises";
 import { resolve } from "node:path";
+import type { ExecBackend, ExecBackendResolver } from "@dispatch/exec-backend";
 import type { ToolContract, ToolResult } from "@dispatch/kernel";
 
 interface ValidatedArgs {
@@ -51,11 +51,21 @@ export function validateArgs(args: unknown): ValidatedArgs | { readonly error: s
 }
 
 /**
- * Factory: create a write_file ToolContract bound to a working directory.
- * The working directory is injected so the tool is testable.
+ * Factory: create a write_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 createWriteFileTool(workingDirectory: string): ToolContract {
-	const workdir = resolve(workingDirectory);
+export function createWriteFileTool(deps: {
+	readonly resolveBackend: ExecBackendResolver;
+	readonly workdir?: string;
+}): ToolContract {
+	const workdir = deps.workdir !== undefined ? resolve(deps.workdir) : undefined;
 
 	return {
 		name: "write_file",
@@ -95,22 +105,21 @@ export function createWriteFileTool(workingDirectory: string): ToolContract {
 			const { path: relPath, content, overwrite } = 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);
 
-			// Check existence.
-			let fileExists = false;
-			try {
-				await access(resolvedPath);
-				fileExists = true;
-			} catch (err: unknown) {
-				const code = (err as NodeJS.ErrnoException).code;
-				if (code !== "ENOENT") {
-					return {
-						content: `Error checking file: ${err instanceof Error ? err.message : String(err)}`,
-						isError: true,
-					};
-				}
-			}
+			const backend: ExecBackend = deps.resolveBackend(ctx.computerId);
+
+			// Check existence. `backend.exists` never throws — it returns false
+			// when the path is missing — so the old try/catch around `access`
+			// collapses to a single boolean read.
+			const fileExists = await backend.exists(resolvedPath);
 
 			// Pure decision.
 			const decision = decideOverwrite(fileExists, overwrite);
@@ -118,10 +127,13 @@ export function createWriteFileTool(workingDirectory: string): ToolContract {
 				return { content: decision.error, isError: true };
 			}
 
-			// Verify it's not a directory.
+			// Verify it's not a directory. `backend.stat` returns a
+			// `{ isFile, isDirectory }` result; only reached when the file
+			// exists, so an ENOENT here is a lost race left to propagate
+			// (same as the prior uncaught `stat` call).
 			if (fileExists) {
-				const pathStat = await stat(resolvedPath);
-				if (pathStat.isDirectory()) {
+				const pathStat = await backend.stat(resolvedPath);
+				if (pathStat.isDirectory) {
 					return {
 						content: `Error: "${relPath}" is a directory, not a file.`,
 						isError: true,
@@ -129,9 +141,11 @@ export function createWriteFileTool(workingDirectory: string): ToolContract {
 				}
 			}
 
-			// Write the file.
+			// Write the file. LocalExecBackend throws node:fs-style errors
+			// carrying a `.code` (e.g. ENOENT when the parent dir is missing);
+			// the catch surfaces the message verbatim.
 			try {
-				await writeFile(resolvedPath, content, "utf8");
+				await backend.writeFile(resolvedPath, content);
 			} catch (err: unknown) {
 				return {
 					content: `Error writing file: ${err instanceof Error ? err.message : String(err)}`,