feat(KB-032): add Planning Mode for AI-assisted task creation

- Add PlanningModeModal component with interactive task planning UI
- Implement backend planning API with /api/planning endpoints
- Add PlanningSession class for managing planning state
- Integrate planning mode into dashboard with header button
- Add comprehensive tests for planning components and API routes
- Include AI agent structure for future planning automation
- Update README with Planning Mode documentation

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

@@ -152,4 +152,23 @@ describe("Header", () => {
       expect(screen.getByTitle("Start AI engine")).toBeDefined();
     });
   });
+
+  describe("planning button", () => {
+    it("renders planning button with correct title", () => {
+      renderHeader({ onOpenPlanning: vi.fn() });
+      expect(screen.getByTitle("Create a task with AI planning")).toBeDefined();
+    });
+
+    it("calls onOpenPlanning when planning button is clicked", () => {
+      const onOpenPlanning = vi.fn();
+      renderHeader({ onOpenPlanning });
+      fireEvent.click(screen.getByTitle("Create a task with AI planning"));
+      expect(onOpenPlanning).toHaveBeenCalled();
+    });
+
+    it("has correct data-testid for testing", () => {
+      renderHeader({ onOpenPlanning: vi.fn() });
+      expect(screen.getByTestId("planning-btn")).toBeDefined();
+    });
+  });
 });