feat(KB-247): add subtask breakdown dialog for AI-assisted task creation

- Add backend subtask breakdown session management and API endpoints
- Create SubtaskBreakdownModal component for generating AI-suggested subtasks
- Integrate subtask breakdown into QuickEntryBox and NewTaskModal flows
- Add frontend API client functions for breakdown sessions and subtask creation
- Add tests for SubtaskBreakdownModal and backend routes
- Create changeset documenting the subtask breakdown feature

packages/dashboard/app/components/SubtaskBreakdownModal.test.tsx

@@ -0,0 +1,118 @@
+import { describe, it, expect, vi, beforeEach, afterEach } from "vitest";
+import { render, screen, fireEvent, waitFor } from "@testing-library/react";
+import { SubtaskBreakdownModal } from "./SubtaskBreakdownModal";
+
+const mockStartSubtaskBreakdown = vi.fn();
+const mockConnectSubtaskStream = vi.fn();
+const mockCreateTasksFromBreakdown = vi.fn();
+const mockCancelSubtaskBreakdown = vi.fn();
+
+vi.mock("../api", () => ({
+  startSubtaskBreakdown: (...args: any[]) => mockStartSubtaskBreakdown(...args),
+  connectSubtaskStream: (...args: any[]) => mockConnectSubtaskStream(...args),
+  createTasksFromBreakdown: (...args: any[]) => mockCreateTasksFromBreakdown(...args),
+  cancelSubtaskBreakdown: (...args: any[]) => mockCancelSubtaskBreakdown(...args),
+}));
+
+const SAMPLE_SUBTASKS = [
+  { id: "subtask-1", title: "First", description: "Do first", suggestedSize: "S" as const, dependsOn: [] },
+  { id: "subtask-2", title: "Second", description: "Do second", suggestedSize: "M" as const, dependsOn: ["subtask-1"] },
+];
+
+describe("SubtaskBreakdownModal", () => {
+  const onClose = vi.fn();
+  const onTasksCreated = vi.fn();
+  let streamHandlers: any;
+
+  beforeEach(() => {
+    vi.clearAllMocks();
+    streamHandlers = undefined;
+    mockStartSubtaskBreakdown.mockResolvedValue({ sessionId: "session-123" });
+    mockConnectSubtaskStream.mockImplementation((_sessionId, handlers) => {
+      streamHandlers = handlers;
+      return { close: vi.fn(), isConnected: () => true };
+    });
+    mockCreateTasksFromBreakdown.mockResolvedValue({ tasks: [{ id: "KB-101" }, { id: "KB-102" }] });
+    mockCancelSubtaskBreakdown.mockResolvedValue(undefined);
+    vi.stubGlobal("confirm", vi.fn(() => true));
+  });
+
+  afterEach(() => {
+    vi.unstubAllGlobals();
+  });
+
+  function renderModal() {
+    return render(
+      <SubtaskBreakdownModal
+        isOpen={true}
+        onClose={onClose}
+        initialDescription="Build a complex feature"
+        onTasksCreated={onTasksCreated}
+      />,
+    );
+  }
+
+  it("shows generating state after auto-start", async () => {
+    renderModal();
+    await waitFor(() => expect(mockStartSubtaskBreakdown).toHaveBeenCalledWith("Build a complex feature"));
+    expect(await screen.findByText("AI is generating subtasks...")).toBeInTheDocument();
+  });
+
+  it("renders editable subtasks when stream returns items", async () => {
+    renderModal();
+    await waitFor(() => expect(streamHandlers).toBeDefined());
+    streamHandlers.onSubtasks(SAMPLE_SUBTASKS);
+    expect(await screen.findByDisplayValue("First")).toBeInTheDocument();
+    expect(screen.getByDisplayValue("Do second")).toBeInTheDocument();
+  });
+
+  it("adds and removes subtasks", async () => {
+    renderModal();
+    await waitFor(() => expect(streamHandlers).toBeDefined());
+    streamHandlers.onSubtasks([SAMPLE_SUBTASKS[0]]);
+
+    fireEvent.click(await screen.findByText("Add subtask"));
+    expect(screen.getAllByText(/subtask-/i).length).toBeGreaterThan(1);
+
+    fireEvent.click(screen.getByText(/Remove/));
+    await waitFor(() => expect(screen.queryByDisplayValue("First")).not.toBeInTheDocument());
+  });
+
+  it("changes size and dependency selection", async () => {
+    renderModal();
+    await waitFor(() => expect(streamHandlers).toBeDefined());
+    streamHandlers.onSubtasks(SAMPLE_SUBTASKS);
+
+    fireEvent.click(await screen.findAllByText("L").then((buttons) => buttons[0]!));
+    fireEvent.click(screen.getByLabelText(/subtask-1/i, { selector: 'input[type="checkbox"]' }));
+    expect(screen.getByText("subtask-1")).toBeInTheDocument();
+  });
+
+  it("saves via API with edited data", async () => {
+    renderModal();
+    await waitFor(() => expect(streamHandlers).toBeDefined());
+    streamHandlers.onSubtasks(SAMPLE_SUBTASKS);
+
+    const titleInputs = await screen.findAllByRole("textbox");
+    fireEvent.change(titleInputs[0], { target: { value: "Updated first" } });
+    fireEvent.click(screen.getByText("Create Tasks"));
+
+    await waitFor(() => expect(mockCreateTasksFromBreakdown).toHaveBeenCalled());
+    expect(onTasksCreated).toHaveBeenCalled();
+    expect(onClose).toHaveBeenCalled();
+  });
+
+  it("cancel closes modal", async () => {
+    renderModal();
+    await waitFor(() => expect(mockStartSubtaskBreakdown).toHaveBeenCalled());
+    fireEvent.click(await screen.findByLabelText("Close"));
+    await waitFor(() => expect(onClose).toHaveBeenCalled());
+  });
+
+  it("escape closes modal", async () => {
+    renderModal();
+    await waitFor(() => expect(mockStartSubtaskBreakdown).toHaveBeenCalled());
+    fireEvent.keyDown(document, { key: "Escape" });
+    await waitFor(() => expect(onClose).toHaveBeenCalled());
+  });
+});