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/db/generated/models.go

@@ -84,6 +84,7 @@ type Comment struct {
 	Type       string             `json:"type"`
 	CreatedAt  pgtype.Timestamptz `json:"created_at"`
 	UpdatedAt  pgtype.Timestamptz `json:"updated_at"`
+	ParentID   pgtype.UUID        `json:"parent_id"`
 }
 
 type DaemonConnection struct {