feat(KB-040): add break-into-subtasks toggle for task creation

- Add breakIntoSubtasks flag to core Task model and persist to store
- Plumb subtask flag through dashboard API routes with validation
- Add subtask toggle UI to inline task creation card
- Update triage agent to handle automatic subtask breakdown when enabled
- Add comprehensive tests for store, API routes, and UI components
- Include changeset for patch release documenting the new feature

packages/dashboard/src/routes.test.ts

@@ -150,6 +150,78 @@ describe("GET /tasks/:id", () => {
   });
 });
 
+describe("POST /tasks", () => {
+  let store: TaskStore;
+
+  beforeEach(() => {
+    store = createMockStore();
+  });
+
+  function buildApp() {
+    const app = express();
+    app.use(express.json());
+    app.use("/api", createApiRoutes(store));
+    return app;
+  }
+
+  it("creates a task and forwards breakIntoSubtasks", async () => {
+    const createdTask = {
+      ...FAKE_TASK_DETAIL,
+      column: "triage",
+      breakIntoSubtasks: true,
+    };
+    (store.createTask as ReturnType<typeof vi.fn>).mockResolvedValue(createdTask);
+
+    const res = await REQUEST(
+      buildApp(),
+      "POST",
+      "/api/tasks",
+      JSON.stringify({
+        description: "Big initiative",
+        breakIntoSubtasks: true,
+      }),
+      { "Content-Type": "application/json" },
+    );
+
+    expect(res.status).toBe(201);
+    expect(store.createTask).toHaveBeenCalledWith({
+      title: undefined,
+      description: "Big initiative",
+      column: undefined,
+      dependencies: undefined,
+      breakIntoSubtasks: true,
+    });
+  });
+
+  it("returns 400 when description is missing", async () => {
+    const res = await REQUEST(
+      buildApp(),
+      "POST",
+      "/api/tasks",
+      JSON.stringify({ breakIntoSubtasks: true }),
+      { "Content-Type": "application/json" },
+    );
+
+    expect(res.status).toBe(400);
+    expect(res.body.error).toContain("description is required");
+    expect(store.createTask).not.toHaveBeenCalled();
+  });
+
+  it("returns 400 when breakIntoSubtasks is not a boolean", async () => {
+    const res = await REQUEST(
+      buildApp(),
+      "POST",
+      "/api/tasks",
+      JSON.stringify({ description: "Big initiative", breakIntoSubtasks: "yes" }),
+      { "Content-Type": "application/json" },
+    );
+
+    expect(res.status).toBe(400);
+    expect(res.body.error).toContain("breakIntoSubtasks must be a boolean");
+    expect(store.createTask).not.toHaveBeenCalled();
+  });
+});
+
 describe("POST /tasks/:id/retry", () => {
   let store: TaskStore;
 

packages/dashboard/src/routes.ts

@@ -610,16 +610,21 @@ export function createApiRoutes(store: TaskStore, options?: ServerOptions): Rout
   // Create task
   router.post("/tasks", async (req, res) => {
     try {
-      const { title, description, column, dependencies } = req.body;
+      const { title, description, column, dependencies, breakIntoSubtasks } = req.body;
       if (!description || typeof description !== "string") {
         res.status(400).json({ error: "description is required" });
         return;
       }
+      if (breakIntoSubtasks !== undefined && typeof breakIntoSubtasks !== "boolean") {
+        res.status(400).json({ error: "breakIntoSubtasks must be a boolean" });
+        return;
+      }
       const task = await store.createTask({
         title,
         description,
         column,
         dependencies,
+        breakIntoSubtasks,
       });
       res.status(201).json(task);
     } catch (err: any) {