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, 5 insertions, 10 deletions

diff --git a/packages/tool-shell/src/shell.ts b/packages/tool-shell/src/shell.ts
index cc76bca..dac7fab 100644
--- a/packages/tool-shell/src/shell.ts
+++ b/packages/tool-shell/src/shell.ts
@@ -1,4 +1,5 @@
 import { resolve } from "node:path";
+import type { ExecBackendResolver } from "@dispatch/exec-backend";
 import type { ToolContract, ToolExecuteContext, ToolResult } from "@dispatch/kernel";
 
 const DEFAULT_TIMEOUT = 120_000;
@@ -15,14 +16,6 @@ export interface SpawnResult {
 	readonly aborted: boolean;
 }
 
-export type SpawnShell = (params: {
-	readonly command: string;
-	readonly cwd: string;
-	readonly signal: AbortSignal;
-	readonly timeout: number;
-	readonly onOutput: (data: string, stream: "stdout" | "stderr") => void;
-}) => Promise<SpawnResult>;
-
 export function validateArgs(args: unknown): ValidatedArgs | { readonly error: string } {
 	if (args === null || args === undefined || typeof args !== "object") {
 		return { error: "Error: Arguments must be an object." };
@@ -88,7 +81,7 @@ export function buildResult(params: {
 
 export function createRunShellTool(deps: {
 	readonly workdir: string;
-	readonly spawn: SpawnShell;
+	readonly resolveBackend: ExecBackendResolver;
 	readonly outputCap?: number;
 }): ToolContract {
 	const workdir = resolve(deps.workdir);
@@ -139,10 +132,12 @@ export function createRunShellTool(deps: {
 				output += data;
 			};
 
+			const backend = deps.resolveBackend(ctx.computerId);
+
 			let spawnResult: SpawnResult;
 
 			try {
-				spawnResult = await deps.spawn({
+				spawnResult = await backend.spawn({
 					command,
 					cwd: effectiveCwd,
 					signal: ctx.signal,