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>

apps/web/shared/api/client.ts

@@ -30,6 +30,7 @@ import type {
   CreatePersonalAccessTokenResponse,
   RuntimeUsage,
   RuntimePing,
+  TimelineEntry,
 } from "@/shared/types";
 import { type Logger, noopLogger } from "@/shared/logger";
 
@@ -183,13 +184,21 @@ export class ApiClient {
     return this.fetch(`/api/issues/${issueId}/comments`);
   }
 
-  async createComment(issueId: string, content: string, type?: string): Promise<Comment> {
+  async createComment(issueId: string, content: string, type?: string, parentId?: string): Promise<Comment> {
     return this.fetch(`/api/issues/${issueId}/comments`, {
       method: "POST",
-      body: JSON.stringify({ content, type: type ?? "comment" }),
+      body: JSON.stringify({
+        content,
+        type: type ?? "comment",
+        ...(parentId ? { parent_id: parentId } : {}),
+      }),
     });
   }
 
+  async listTimeline(issueId: string): Promise<TimelineEntry[]> {
+    return this.fetch(`/api/issues/${issueId}/timeline`);
+  }
+
   async updateComment(commentId: string, content: string): Promise<Comment> {
     return this.fetch(`/api/comments/${commentId}`, {
       method: "PUT",