feat(activity): unified activity timeline with comment reply support

Replace the comment-only list with a Linear-style unified timeline that
interleaves field changes and comments chronologically.

Backend:
- activity_listeners.go: records field changes (status, assignee, description,
  task completed/failed) to activity_log table on domain events
- Timeline API: GET /api/issues/{id}/timeline merges activity_log + comments
  sorted by created_at
- Comment reply: parent_id column + handler support for threading

Frontend:
- Unified timeline replaces comment list: activity entries as compact muted
  lines, comments as Card components with reply threading
- Filter toggle (All / Comments / Activity)
- Reply UI: inline editor under comments with Cancel/Reply buttons
- Real-time sync for activity:created + comment events
- 10 new Go tests, all passing

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/pkg/protocol/events.go

@@ -43,6 +43,9 @@ const (
 	EventSubscriberAdded   = "subscriber:added"
 	EventSubscriberRemoved = "subscriber:removed"
 
+	// Activity events
+	EventActivityCreated = "activity:created"
+
 	// Daemon events
 	EventDaemonHeartbeat = "daemon:heartbeat"
 	EventDaemonRegister  = "daemon:register"