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, 2 deletions

diff --git a/packages/tool-shell/src/extension.ts b/packages/tool-shell/src/extension.ts
index 1a89de0..984263e 100644
--- a/packages/tool-shell/src/extension.ts
+++ b/packages/tool-shell/src/extension.ts
@@ -1,6 +1,6 @@
+import { execBackendHandle } from "@dispatch/exec-backend";
 import type { Extension } from "@dispatch/kernel";
 import { createRunShellTool } from "./shell.js";
-import { realSpawn } from "./spawn.js";
 
 export const extension: Extension = {
 	manifest: {
@@ -12,8 +12,11 @@ export const extension: Extension = {
 		activation: "eager",
 		capabilities: { shell: true },
 		contributes: { tools: ["run_shell"] },
+		// Host activates exec-backend first → host.getService at activation is safe.
+		dependsOn: ["exec-backend"],
 	},
 	activate(host) {
-		host.defineTool(createRunShellTool({ workdir: process.cwd(), spawn: realSpawn }));
+		const resolveBackend = host.getService(execBackendHandle);
+		host.defineTool(createRunShellTool({ workdir: process.cwd(), resolveBackend }));
 	},
 };