feat(KB-175): move file browser to header with workspace support

- Add workspace file APIs and hooks for session-scoped file browsing
- Create WorkspaceSelector component for switching between workspaces
- Update FileBrowserModal with workspace mode and improved UX
- Add files button to header with workspace-aware file browsing
- Add session files indicator to task cards for quick file access
- Include comprehensive tests for all new components and hooks

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

@@ -0,0 +1,74 @@
+import { describe, it, expect, vi, beforeEach, afterEach } from "vitest";
+import { renderHook, waitFor } from "@testing-library/react";
+import { useWorkspaces } from "../useWorkspaces";
+import * as api from "../../api";
+
+vi.mock("../../api", () => ({
+  fetchWorkspaces: vi.fn(),
+}));
+
+const mockFetchWorkspaces = vi.mocked(api.fetchWorkspaces);
+
+describe("useWorkspaces", () => {
+  beforeEach(() => {
+    vi.useFakeTimers();
+    mockFetchWorkspaces.mockReset();
+  });
+
+  afterEach(() => {
+    vi.useRealTimers();
+    vi.clearAllMocks();
+  });
+
+  it("loads project and task workspaces", async () => {
+    mockFetchWorkspaces.mockResolvedValueOnce({
+      project: "/Users/test/repo",
+      tasks: [{ id: "KB-123", title: "Feature", worktree: "/Users/test/.worktrees/kb-123" }],
+    });
+
+    const { result } = renderHook(() => useWorkspaces());
+
+    await waitFor(() => expect(result.current.loading).toBe(false));
+
+    expect(result.current.projectName).toBe("repo");
+    expect(result.current.workspaces).toEqual([
+      {
+        id: "KB-123",
+        label: "KB-123",
+        title: "Feature",
+        worktree: "/Users/test/.worktrees/kb-123",
+        kind: "task",
+      },
+    ]);
+  });
+
+  it("polls for workspace updates", async () => {
+    mockFetchWorkspaces
+      .mockResolvedValueOnce({ project: "/repo", tasks: [] })
+      .mockResolvedValueOnce({
+        project: "/repo",
+        tasks: [{ id: "KB-200", title: "Later", worktree: "/repo/.worktrees/kb-200" }],
+      });
+
+    const { result } = renderHook(() => useWorkspaces());
+
+    await waitFor(() => expect(result.current.loading).toBe(false));
+    expect(result.current.workspaces).toEqual([]);
+
+    await vi.advanceTimersByTimeAsync(10000);
+
+    await waitFor(() => expect(result.current.workspaces).toHaveLength(1));
+    expect(mockFetchWorkspaces).toHaveBeenCalledTimes(2);
+  });
+
+  it("surfaces fetch errors", async () => {
+    mockFetchWorkspaces.mockRejectedValueOnce(new Error("Failed to load workspaces"));
+
+    const { result } = renderHook(() => useWorkspaces());
+
+    await waitFor(() => expect(result.current.loading).toBe(false));
+
+    expect(result.current.error).toBe("Failed to load workspaces");
+    expect(result.current.workspaces).toEqual([]);
+  });
+});