feat: Add persistent chat sessions for iterative phase/task refinement

Introduces a chat loop where users send instructions to an agent that
applies changes (create/update/delete phases, tasks, pages) and stays
alive for follow-up messages. Includes schema + migration, repository
layer, chat prompt, file-io action field extension, output handler chat
mode, revert support for deletes, tRPC procedures, events, frontend
slide-over UI with inline changeset display and revert, and docs.

docs/dispatch-events.md

@@ -11,7 +11,7 @@
 - **Adapter**: `TypedEventBus` using Node.js `EventEmitter`
 - All events implement `BaseEvent { type, timestamp, payload }`
 
-### Event Types (52)
+### Event Types (54)
 
 | Category | Events | Count |
 |----------|--------|-------|
@@ -26,6 +26,7 @@
 | **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:created` triggers auto-resume of idle target agents via `resumeForConversation()` |
+| **Chat** | `chat:message_created`, `chat:session_closed` | 2 | Chat session lifecycle events |
 | **Log** | `log:entry` | 1 |
 
 ### Key Event Payloads
@@ -34,7 +35,7 @@
 AgentSpawnedEvent    { agentId, name, taskId, worktreeId, provider }
 AgentStoppedEvent    { agentId, name, taskId, reason }
   // reason: 'user_requested'|'task_complete'|'error'|'waiting_for_input'|
-  //         'context_complete'|'plan_complete'|'detail_complete'|'refine_complete'
+  //         'context_complete'|'plan_complete'|'detail_complete'|'refine_complete'|'chat_complete'
 AgentWaitingEvent    { agentId, name, taskId, sessionId, questions[] }
 AgentOutputEvent     { agentId, stream, data }
 TaskCompletedEvent   { taskId, agentId, success, message }