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/components/__tests__/WorkspaceSelector.test.tsx

@@ -0,0 +1,92 @@
+import { describe, it, expect, vi } from "vitest";
+import { render, screen, within } from "@testing-library/react";
+import userEvent from "@testing-library/user-event";
+import { WorkspaceSelector } from "../WorkspaceSelector";
+import type { WorkspaceInfo } from "../../hooks/useWorkspaces";
+
+const workspaces: WorkspaceInfo[] = [
+  {
+    id: "KB-123",
+    label: "KB-123",
+    title: "Implement a very long task title that should be truncated in the dropdown list",
+    worktree: "/repo/.worktrees/kb-123",
+    kind: "task",
+  },
+  {
+    id: "KB-200",
+    label: "KB-200",
+    title: "Short title",
+    worktree: "/repo/.worktrees/kb-200",
+    kind: "task",
+  },
+];
+
+describe("WorkspaceSelector", () => {
+  it("shows the project name when project root is selected", () => {
+    render(
+      <WorkspaceSelector
+        currentWorkspace="project"
+        projectName="kb"
+        workspaces={workspaces}
+        onSelect={vi.fn()}
+      />,
+    );
+
+    expect(screen.getByRole("button", { name: /kb/i })).toBeInTheDocument();
+  });
+
+  it("opens the menu and highlights the current workspace", async () => {
+    const user = userEvent.setup();
+    render(
+      <WorkspaceSelector
+        currentWorkspace="KB-200"
+        projectName="kb"
+        workspaces={workspaces}
+        onSelect={vi.fn()}
+      />,
+    );
+
+    await user.click(screen.getByRole("button", { name: /KB-200/i }));
+
+    expect(screen.getByText("Project Root")).toBeInTheDocument();
+    const menu = screen.getByRole("listbox", { name: /select workspace/i });
+    const activeOption = within(menu).getByRole("button", { name: /KB-200 Short title/i });
+    expect(activeOption.className).toContain("active");
+  });
+
+  it("calls onSelect and closes when choosing a workspace", async () => {
+    const user = userEvent.setup();
+    const onSelect = vi.fn();
+    render(
+      <WorkspaceSelector
+        currentWorkspace="project"
+        projectName="kb"
+        workspaces={workspaces}
+        onSelect={onSelect}
+      />,
+    );
+
+    await user.click(screen.getByRole("button", { name: /kb/i }));
+    await user.click(screen.getByRole("button", { name: /KB-123/i }));
+
+    expect(onSelect).toHaveBeenCalledWith("KB-123");
+    expect(screen.queryByText("Task Worktrees")).not.toBeInTheDocument();
+  });
+
+  it("truncates long task titles in the dropdown", async () => {
+    const user = userEvent.setup();
+    render(
+      <WorkspaceSelector
+        currentWorkspace="project"
+        projectName="kb"
+        workspaces={workspaces}
+        onSelect={vi.fn()}
+      />,
+    );
+
+    await user.click(screen.getByRole("button", { name: /kb/i }));
+
+    expect(screen.getByText(/Implement a very long task title that shoul/)).toBeInTheDocument();
+    expect(screen.getByText(/…$/)).toBeInTheDocument();
+  });
+});