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/dispatch-events.md

@@ -25,7 +25,7 @@
 | **Worktree** | `worktree:created`, `worktree:removed`, `worktree:merged`, `worktree:conflict` | 4 |
 | **Account** | `account:credentials_refreshed`, `account:credentials_expired`, `account:credentials_validated` | 3 |
 | **Preview** | `preview:building`, `preview:ready`, `preview:stopped`, `preview:failed` | 4 |
-| **Conversation** | `conversation:created`, `conversation:answered` | 2 |
+| **Conversation** | `conversation:created`, `conversation:answered` | 2 | `conversation:created` triggers auto-resume of idle target agents via `resumeForConversation()` |
 | **Log** | `log:entry` | 1 |
 
 ### Key Event Payloads