feat: task comments/discussion system

- New task_comments table (separate from progress notes)
- Backend: GET/POST/DELETE /api/tasks/:id/comments with session + bearer auth
- TaskComments component on TaskPage (full-page view) with markdown support,
  author avatars, delete own comments, 30s polling
- CompactComments in TaskDetailPanel (side panel) with last 3 + expand
- Comment API functions in frontend lib/api.ts

frontend/src/pages/TaskPage.tsx

@@ -5,6 +5,7 @@ import remarkGfm from "remark-gfm";
 import type { Task, TaskStatus, Project } from "../lib/types";
 import { updateTask, fetchProjects, addProgressNote, addSubtask, toggleSubtask, deleteSubtask, deleteTask } from "../lib/api";
 import { useToast } from "../components/Toast";
+import { TaskComments } from "../components/TaskComments";
 
 const priorityColors: Record<string, string> = {
   critical: "bg-red-500 text-white",
@@ -598,6 +599,8 @@ export function TaskPage() {
                 </div>
               )}
             </div>
+            {/* Comments / Discussion */}
+            <TaskComments taskId={task.id} />
           </div>
 
           {/* Sidebar */}