feat(KB-016): add task duplicate command

- Add duplicateTask() method to core store with fresh state reset
- Add CLI command handler and register duplicate subcommand
- Add POST /tasks/:id/duplicate API endpoint to dashboard
- Add comprehensive tests for store, CLI, and API routes
- Add pi extension tool for task duplication
- Add changeset for minor release bump

packages/dashboard/app/api.ts

@@ -63,6 +63,10 @@ export function retryTask(id: string): Promise<Task> {
   return api<Task>(`/tasks/${id}/retry`, { method: "POST" });
 }
 
+export function duplicateTask(id: string): Promise<Task> {
+  return api<Task>(`/tasks/${id}/duplicate`, { method: "POST" });
+}
+
 export function pauseTask(id: string): Promise<Task> {
   return api<Task>(`/tasks/${id}/pause`, { method: "POST" });
 }

packages/dashboard/src/routes.test.ts

@@ -206,6 +206,61 @@ describe("POST /tasks/:id/retry", () => {
   });
 });
 
+describe("POST /tasks/:id/duplicate", () => {
+  let store: TaskStore;
+
+  beforeEach(() => {
+    store = createMockStore({
+      duplicateTask: vi.fn(),
+    });
+  });
+
+  function buildApp() {
+    const app = express();
+    app.use(express.json());
+    app.use("/api", createApiRoutes(store));
+    return app;
+  }
+
+  it("duplicates a task and returns 201 with new task", async () => {
+    const newTask = { ...FAKE_TASK_DETAIL, id: "KB-002", column: "triage" };
+    (store.duplicateTask as ReturnType<typeof vi.fn>).mockResolvedValue(newTask);
+
+    const res = await REQUEST(buildApp(), "POST", "/api/tasks/KB-001/duplicate", JSON.stringify({}), {
+      "Content-Type": "application/json",
+    });
+
+    expect(res.status).toBe(201);
+    expect(res.body.id).toBe("KB-002");
+    expect(res.body.column).toBe("triage");
+    expect(store.duplicateTask).toHaveBeenCalledWith("KB-001");
+  });
+
+  it("returns 404 when source task not found", async () => {
+    const error = new Error("Task not found") as NodeJS.ErrnoException;
+    error.code = "ENOENT";
+    (store.duplicateTask as ReturnType<typeof vi.fn>).mockRejectedValue(error);
+
+    const res = await REQUEST(buildApp(), "POST", "/api/tasks/KB-999/duplicate", JSON.stringify({}), {
+      "Content-Type": "application/json",
+    });
+
+    expect(res.status).toBe(404);
+    expect(res.body.error).toContain("not found");
+  });
+
+  it("returns 500 on unexpected errors", async () => {
+    (store.duplicateTask as ReturnType<typeof vi.fn>).mockRejectedValue(new Error("Database error"));
+
+    const res = await REQUEST(buildApp(), "POST", "/api/tasks/KB-001/duplicate", JSON.stringify({}), {
+      "Content-Type": "application/json",
+    });
+
+    expect(res.status).toBe(500);
+    expect(res.body.error).toContain("Database error");
+  });
+});
+
 describe("PATCH /tasks/:id", () => {
   let store: TaskStore;
 

packages/dashboard/src/routes.ts

@@ -276,6 +276,17 @@ export function createApiRoutes(store: TaskStore, options?: ServerOptions): Rout
     }
   });
 
+  // Duplicate task
+  router.post("/tasks/:id/duplicate", async (req, res) => {
+    try {
+      const newTask = await store.duplicateTask(req.params.id);
+      res.status(201).json(newTask);
+    } catch (err: any) {
+      const status = err.code === "ENOENT" ? 404 : 500;
+      res.status(status).json({ error: err.message });
+    }
+  });
+
   // Upload attachment
   router.post("/tasks/:id/attachments", upload.single("file"), async (req, res) => {
     try {