feat: Add agent spawn infrastructure for errand mode

Implements three primitives needed before errand tRPC procedures can be wired up:

- agentManager.sendUserMessage(agentId, message): resumes an errand agent with a
  raw user message, bypassing the conversations table and conversationResumeLocks.
  Throws on missing agent, invalid status, or absent sessionId.

- writeErrandManifest(options): writes .cw/input/errand.md (YAML frontmatter),
  .cw/input/manifest.json (errandId/agentId/agentName/mode, no files/contextFiles),
  and .cw/expected-pwd.txt to an agent workdir.

- buildErrandPrompt(description): minimal prompt for errand agents; exported from
  prompts/errand.ts and re-exported from prompts/index.ts.

Also fixes a pre-existing TypeScript error in lifecycle/controller.test.ts (missing
backoffMs property in RetryPolicy mock introduced by a concurrent agent commit).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/server/agent/prompts/errand.ts

@@ -0,0 +1,16 @@
+export function buildErrandPrompt(description: string): string {
+  return `You are working on a small, focused change in an isolated worktree.
+
+Description: ${description}
+
+Work interactively with the user. Make only the changes needed to fulfill the description.
+When you are done, write .cw/output/signal.json:
+
+{ "status": "done", "result": { "message": "<one-sentence summary of what you changed>" } }
+
+If you cannot complete the change:
+
+{ "status": "error", "error": "<explanation>" }
+
+Do not create any other output files.`;
+}