feat: Add inter-agent conversation system (listen, ask, answer)

Enables parallel agents to communicate through a CLI-based conversation
mechanism coordinated via tRPC. Agents can ask questions to peers and
receive answers, with target resolution by agent ID, task ID, or phase ID.

docs/dispatch-events.md

@@ -25,6 +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 |
 | **Log** | `log:entry` | 1 |
 
 ### Key Event Payloads