feat(agent): stream live agent output to issue detail page

When an agent is working on an issue, users can now see real-time output
in the issue detail page instead of waiting for completion.

Backend:
- Add task_message table and migration for persisting agent messages
- Add POST /api/daemon/tasks/{id}/messages endpoint for daemon to report
  structured messages (tool_use, tool_result, text, error) in batches
- Add GET /api/daemon/tasks/{id}/messages for catch-up after reconnect
- Add GET /api/issues/{id}/active-task to check for running tasks
- Broadcast task:message events via WebSocket
- Daemon forwards agent session messages with 500ms text throttling

Frontend:
- Add AgentLiveCard component showing live tool calls, text output,
  and progress indicators with auto-scroll
- Wire into issue detail timeline with WS subscription and HTTP catch-up
- Card appears when agent is working, disappears on completion/failure

server/cmd/server/router.go

@@ -99,6 +99,8 @@ func NewRouter(pool *pgxpool.Pool, hub *realtime.Hub, bus *events.Bus) chi.Route
 		r.Post("/tasks/{taskId}/progress", h.ReportTaskProgress)
 		r.Post("/tasks/{taskId}/complete", h.CompleteTask)
 		r.Post("/tasks/{taskId}/fail", h.FailTask)
+		r.Post("/tasks/{taskId}/messages", h.ReportTaskMessages)
+		r.Get("/tasks/{taskId}/messages", h.ListTaskMessages)
 	})
 
 	// Protected API routes
@@ -164,6 +166,7 @@ func NewRouter(pool *pgxpool.Pool, hub *realtime.Hub, bus *events.Bus) chi.Route
 					r.Get("/subscribers", h.ListIssueSubscribers)
 					r.Post("/subscribe", h.SubscribeToIssue)
 					r.Post("/unsubscribe", h.UnsubscribeFromIssue)
+					r.Get("/active-task", h.GetActiveTaskForIssue)
 				})
 			})