feat(KB-069): add Archive All Done feature

- Add archiveAllDone method to TaskStore with filtering and batch archive
- Add POST /tasks/archive-all-done API endpoint with tests
- Add useTasks hook support and API function for archiveAllDone
- Add Archive All button to done column header in Board UI
- Wire up onArchiveAllDone through Board component hierarchy

packages/dashboard/app/hooks/__tests__/useTasks.test.ts

@@ -30,10 +30,14 @@ vi.mock("../../api", () => ({
   mergeTask: vi.fn(),
   retryTask: vi.fn(),
   updateTask: vi.fn(),
+  archiveTask: vi.fn(),
+  unarchiveTask: vi.fn(),
+  archiveAllDone: vi.fn(),
 }));
 
 const mockFetchTasks = vi.mocked(api.fetchTasks);
 const mockUpdateTask = vi.mocked(api.updateTask);
+const mockArchiveAllDone = vi.mocked(api.archiveAllDone);
 
 // Mock EventSource
 class MockEventSource {
@@ -643,4 +647,57 @@ describe("useTasks", () => {
       expect(result.current.tasks[0].dependencies).toEqual(["KB-002", "KB-003"]);
     });
   });
+
+  describe("archiveAllDone", () => {
+    it("archives all done tasks and updates local state", async () => {
+      const doneTasks = [
+        createMockTask({ id: "KB-001", column: "done" as Column }),
+        createMockTask({ id: "KB-002", column: "done" as Column }),
+      ];
+      const todoTask = createMockTask({ id: "KB-003", column: "todo" as Column });
+      mockFetchTasks.mockResolvedValueOnce([...doneTasks, todoTask]);
+
+      const archivedTasks = [
+        createMockTask({ id: "KB-001", column: "archived" as Column }),
+        createMockTask({ id: "KB-002", column: "archived" as Column }),
+      ];
+      mockArchiveAllDone.mockResolvedValueOnce(archivedTasks);
+
+      const { result } = renderHook(() => useTasks());
+
+      await waitFor(() => {
+        expect(result.current.tasks).toHaveLength(3);
+      });
+
+      await act(async () => {
+        await result.current.archiveAllDone();
+      });
+
+      expect(mockArchiveAllDone).toHaveBeenCalled();
+      // Done tasks should be archived
+      expect(result.current.tasks.find((t) => t.id === "KB-001")?.column).toBe("archived");
+      expect(result.current.tasks.find((t) => t.id === "KB-002")?.column).toBe("archived");
+      // Todo task should remain unchanged
+      expect(result.current.tasks.find((t) => t.id === "KB-003")?.column).toBe("todo");
+    });
+
+    it("returns empty array when no done tasks exist", async () => {
+      const todoTask = createMockTask({ id: "KB-001", column: "todo" as Column });
+      mockFetchTasks.mockResolvedValueOnce([todoTask]);
+      mockArchiveAllDone.mockResolvedValueOnce([]);
+
+      const { result } = renderHook(() => useTasks());
+
+      await waitFor(() => {
+        expect(result.current.tasks).toHaveLength(1);
+      });
+
+      const archived = await act(async () => {
+        return await result.current.archiveAllDone();
+      });
+
+      expect(archived).toEqual([]);
+      expect(result.current.tasks[0].column).toBe("todo");
+    });
+  });
 });

packages/dashboard/app/hooks/useTasks.ts

@@ -214,5 +214,18 @@ export function useTasks() {
     return task;
   }, []);
 
-  return { tasks, createTask, moveTask, deleteTask, mergeTask, retryTask, duplicateTask, updateTask, archiveTask, unarchiveTask };
+  const archiveAllDone = useCallback(async (): Promise<Task[]> => {
+    const archived = await api.archiveAllDone();
+    const normalized = archived.map(normalizeTask);
+    // Update local state by mapping over tasks and updating archived ones
+    setTasks((prev) =>
+      prev.map((t) => {
+        const updated = normalized.find((archived) => archived.id === t.id);
+        return updated || t;
+      })
+    );
+    return normalized;
+  }, []);
+
+  return { tasks, createTask, moveTask, deleteTask, mergeTask, retryTask, duplicateTask, updateTask, archiveTask, unarchiveTask, archiveAllDone };
 }