feat: Auto-resume idle agents for inter-agent conversations

When an agent asks a question via `cw ask` targeting an idle agent,
the conversation router now auto-resumes the idle agent's session so
it can answer. Previously, questions to idle agents sat unanswered
forever because target resolution only matched running agents.

Changes:
- Add `resumeForConversation()` to AgentManager interface and implement
  on MultiProviderAgentManager (mirrors resumeForCommit pattern)
- Relax createConversation target resolution: prefer running, fall back
  to idle (was running-only)
- Trigger auto-resume after conversation creation for idle targets
- Add concurrency lock (conversationResumeLocks Set) to prevent
  double-resume race conditions

docs/agent.md

@@ -115,6 +115,25 @@ cw account add --token <token> --email user@example.com
 
 Stored as `credentials: {"claudeAiOauth":{"accessToken":"<token>"}}` and `configJson: {"hasCompletedOnboarding":true}`.
 
+## 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.
+
+### Flow
+
+1. `createConversation` tRPC procedure creates the conversation record
+2. Target resolution prefers `running` agents, falls back to `idle` (previously only matched `running`)
+3. After creation, checks if target agent is idle → calls `agentManager.resumeForConversation()`
+4. Agent resumes with a prompt to: answer via `cw answer`, drain pending conversations via `cw listen`, then complete
+
+### Guards
+
+- Agent must be `idle` status with a valid `sessionId`
+- Provider must support resume (`resumeStyle !== 'none'`)
+- Worktree must still exist (`existsSync` check)
+- In-memory `conversationResumeLocks` Set prevents double-resume race when multiple conversations arrive simultaneously
+- Resume failure is caught and logged — conversation is always created even if resume fails
+
 ## Auto-Cleanup & Commit Retries
 
 After an agent completes (status → `idle`), `tryAutoCleanup` checks if its project worktrees have uncommitted changes: