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>

docs/agent.md

@@ -11,7 +11,7 @@
 | `process-manager.ts` | `AgentProcessManager` — worktree creation, command building, detached spawn |
 | `output-handler.ts` | `OutputHandler` — JSONL stream parsing, completion detection, proposal creation, task dedup, task dependency persistence |
 | `file-tailer.ts` | `FileTailer` — watches output files, fires parser + raw content callbacks |
-| `file-io.ts` | Input/output file I/O: frontmatter writing, signal.json reading, tiptap conversion. Output files support `action` field (create/update/delete) for chat mode CRUD. |
+| `file-io.ts` | Input/output file I/O: frontmatter writing, signal.json reading, tiptap conversion. Output files support `action` field (create/update/delete) for chat mode CRUD. Includes `writeErrandManifest()` for errand agent input files. |
 | `markdown-to-tiptap.ts` | Markdown to Tiptap JSON conversion using MarkdownManager |
 | `index.ts` | Public exports, `ClaudeAgentManager` deprecated alias |
 
@@ -24,7 +24,7 @@
 | `accounts/` | Account discovery, config dir setup, credential management, usage API |
 | `credentials/` | `AccountCredentialManager` — credential injection per account |
 | `lifecycle/` | `LifecycleController` — retry policy, signal recovery, missing signal instructions |
-| `prompts/` | Mode-specific prompt builders (execute, discuss, plan, detail, refine, chat, conflict-resolution) + shared blocks (test integrity, deviation rules, git workflow, session startup, progress tracking) + inter-agent communication instructions |
+| `prompts/` | Mode-specific prompt builders (execute, discuss, plan, detail, refine, chat, conflict-resolution, errand) + shared blocks (test integrity, deviation rules, git workflow, session startup, progress tracking) + inter-agent communication instructions |
 
 ## Key Flows
 
@@ -115,6 +115,30 @@ cw account add --token <token> --email user@example.com
 
 Stored as `credentials: {"claudeAiOauth":{"accessToken":"<token>"}}` and `configJson: {"hasCompletedOnboarding":true}`.
 
+## Errand Agent Support
+
+### `sendUserMessage(agentId, message)`
+
+Delivers a user message directly to a running or idle errand agent without going through the conversations table. Used by the `errand.sendMessage` tRPC procedure.
+
+**Steps**: look up agent → validate status (`running`|`idle`) → validate `sessionId` → clear signal.json → update status to `running` → build resume command → stop active tailer/poll → spawn detached → start polling.
+
+**Key difference from `resumeForConversation`**: no `conversationResumeLocks`, no conversations table entry, raw message passed as resume prompt.
+
+### `writeErrandManifest(options)`
+
+Writes errand input files to `<agentWorkdir>/.cw/input/`:
+
+- `errand.md` — YAML frontmatter with `id`, `description`, `branch`, `project`
+- `manifest.json` — `{ errandId, agentId, agentName, mode: "errand" }` (no `files`/`contextFiles` arrays)
+- `expected-pwd.txt` — the agent workdir path
+
+Written in order: `errand.md` first, `manifest.json` last (same discipline as `writeInputFiles`).
+
+### `buildErrandPrompt(description)`
+
+Builds the initial prompt for errand agents. Exported from `prompts/errand.ts` and re-exported from `prompts/index.ts`. The prompt instructs the agent to make only the changes needed for the description and write `signal.json` when done.
+
 ## Auto-Resume for Conversations
 
 When Agent A asks Agent B a question via `cw ask` and Agent B is idle, the conversation router automatically resumes Agent B's session. This mirrors the `resumeForCommit()` pattern.