Update optimistic timestamp to enable smart merge strategy

- Set updated_at to current ISO timestamp during optimistic updates
- Ensures smart merge logic triggers when server responses are stale
- Fixes CodeRabbit review suggestion about timestamp comparison effectiveness

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

archon-ui-main/src/features/projects/tasks/hooks/useTaskQueries.ts

@@ -112,7 +112,10 @@ export function useUpdateTask(projectId: string) {
       // Optimistically update
       queryClient.setQueryData<Task[]>(taskKeys.all(projectId), (old) => {
         if (!old) return old;
-        return old.map((task) => (task.id === taskId ? { ...task, ...updates } : task));
+        const nowIso = new Date().toISOString();
+        return old.map((task) =>
+          task.id === taskId ? { ...task, ...updates, updated_at: nowIso } : task,
+        );
       });
 
       return { previousTasks };