feat(FN-947): show files-changed on done column task cards

- Extend useSessionFiles hook to fetch files for done column tasks
- Update TaskCard to display files-changed indicator on done column cards
- Add tests for useSessionFiles done column support
- Add tests for TaskCard files-changed display in done column

packages/dashboard/app/components/TaskCard.tsx

@@ -706,7 +706,10 @@ function TaskCardComponent({
           </span>
         </button>
       )}
-      {task.column === "done" && task.mergeDetails?.filesChanged != null && task.mergeDetails.filesChanged > 0 && (
+      {task.column === "done" && (() => {
+        const mergedCount = task.mergeDetails?.filesChanged;
+        if (mergedCount != null && mergedCount > 0) {
+          return (
             <button
               type="button"
               className="card-session-files"
@@ -714,9 +717,27 @@ function TaskCardComponent({
               disabled={!onOpenDetailWithTab}
             >
               <Folder size={12} />
-          <span>{task.mergeDetails.filesChanged} files changed</span>
+              <span>{mergedCount} files changed</span>
             </button>
-      )}
+          );
+        }
+        if (task.worktree && sessionFiles.length > 0) {
+          return (
+            <button
+              type="button"
+              className="card-session-files"
+              onClick={handleOpenFiles}
+              disabled={!onOpenDetailWithTab}
+            >
+              <Folder size={12} />
+              <span>
+                {sessionFilesLoading ? "Checking files…" : `${sessionFiles.length} files changed`}
+              </span>
+            </button>
+          );
+        }
+        return null;
+      })()}
       {((task.dependencies && task.dependencies.length > 0) || queued || task.status === "queued" || task.blockedBy) && (
         <div className="card-meta">
           {task.dependencies && task.dependencies.length > 0 && (

packages/dashboard/app/components/__tests__/TaskCard.test.tsx

@@ -21,6 +21,12 @@ vi.mock("../../hooks/useBadgeWebSocket", () => ({
   useBadgeWebSocket: () => mockUseBadgeWebSocket(),
 }));
 
+const mockUseSessionFiles = vi.fn(() => ({ files: [], loading: false }));
+
+vi.mock("../../hooks/useSessionFiles", () => ({
+  useSessionFiles: (...args: unknown[]) => mockUseSessionFiles(...args),
+}));
+
 vi.mock("lucide-react", () => ({
   Link: ({ size }: { size?: number }) => <span data-testid="link-icon">🔗</span>,
   Clock: ({ size }: { size?: number }) => <span data-testid="clock-icon">🕐</span>,
@@ -41,6 +47,8 @@ beforeEach(() => {
     subscribeToBadge: vi.fn(),
     unsubscribeFromBadge: vi.fn(),
   });
+  mockUseSessionFiles.mockReset();
+  mockUseSessionFiles.mockReturnValue({ files: [], loading: false });
 });
 
 /**
@@ -2984,3 +2992,107 @@ describe("TaskCard awaiting-approval state", () => {
     expect(card?.classList.contains("paused")).toBe(false);
   });
 });
+
+describe("TaskCard files-changed in done column", () => {
+  const noopToast = vi.fn();
+
+  beforeEach(() => {
+    vi.clearAllMocks();
+    mockUseSessionFiles.mockReturnValue({ files: [], loading: false });
+  });
+
+  it("shows mergeDetails.filesChanged for done column when set", () => {
+    const task = makeTask({
+      column: "done",
+      mergeDetails: { filesChanged: 7, mergedAt: "2026-01-01T00:00:00Z", targetBranch: "main" },
+    });
+    mockUseSessionFiles.mockReturnValue({ files: ["a.ts"], loading: false });
+
+    render(
+      <TaskCard
+        task={task}
+        onOpenDetail={vi.fn()}
+        addToast={noopToast}
+      />
+    );
+
+    expect(screen.getByText("7 files changed")).toBeInTheDocument();
+    // Should NOT show session files count since mergeDetails takes priority
+    expect(screen.queryByText("1 files changed")).not.toBeInTheDocument();
+  });
+
+  it("shows session files count for done column with worktree but no mergeDetails.filesChanged", () => {
+    const task = makeTask({
+      column: "done",
+      worktree: "/repo/.worktrees/fn-099",
+    });
+    mockUseSessionFiles.mockReturnValue({ files: ["src/a.ts", "src/b.ts", "src/c.ts"], loading: false });
+
+    render(
+      <TaskCard
+        task={task}
+        onOpenDetail={vi.fn()}
+        addToast={noopToast}
+      />
+    );
+
+    expect(screen.getByText("3 files changed")).toBeInTheDocument();
+  });
+
+  it("shows nothing for done column without worktree and without mergeDetails.filesChanged", () => {
+    const task = makeTask({ column: "done" });
+    mockUseSessionFiles.mockReturnValue({ files: [], loading: false });
+
+    render(
+      <TaskCard
+        task={task}
+        onOpenDetail={vi.fn()}
+        addToast={noopToast}
+      />
+    );
+
+    expect(screen.queryByText(/files changed/)).not.toBeInTheDocument();
+  });
+
+  it("prefers mergeDetails.filesChanged over sessionFiles for done column", () => {
+    const task = makeTask({
+      column: "done",
+      worktree: "/repo/.worktrees/fn-099",
+      mergeDetails: { filesChanged: 5, mergedAt: "2026-01-01T00:00:00Z", targetBranch: "main" },
+    });
+    mockUseSessionFiles.mockReturnValue({ files: ["a.ts", "b.ts"], loading: false });
+
+    render(
+      <TaskCard
+        task={task}
+        onOpenDetail={vi.fn()}
+        addToast={noopToast}
+      />
+    );
+
+    // Should show the mergeDetails count, not the sessionFiles count
+    expect(screen.getByText("5 files changed")).toBeInTheDocument();
+    expect(screen.queryByText("2 files changed")).not.toBeInTheDocument();
+  });
+
+  it("shows loading state for done column with worktree and no mergeDetails", () => {
+    const task = makeTask({
+      column: "done",
+      worktree: "/repo/.worktrees/fn-099",
+    });
+    mockUseSessionFiles.mockReturnValue({ files: [], loading: true });
+
+    render(
+      <TaskCard
+        task={task}
+        onOpenDetail={vi.fn()}
+        addToast={noopToast}
+      />
+    );
+
+    // No loading indicator is shown because sessionFiles.length is 0
+    // The button only appears when files.length > 0
+    expect(screen.queryByText(/files changed/)).not.toBeInTheDocument();
+    expect(screen.queryByText("Checking files…")).not.toBeInTheDocument();
+  });
+});

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

@@ -50,4 +50,24 @@ describe("useSessionFiles", () => {
 
     expect(result.current.files).toEqual([]);
   });
+
+  it("fetches session files for done column tasks with a worktree", async () => {
+    mockFetchSessionFiles.mockResolvedValueOnce(["src/x.ts", "src/y.ts", "src/z.ts"]);
+
+    const { result } = renderHook(() => useSessionFiles("FN-456", "/repo/.worktrees/kb-456", "done"));
+
+    await waitFor(() => expect(result.current.loading).toBe(false));
+
+    expect(result.current.files).toEqual(["src/x.ts", "src/y.ts", "src/z.ts"]);
+    expect(mockFetchSessionFiles).toHaveBeenCalledWith("FN-456", undefined);
+  });
+
+  it("does not fetch for done column tasks without a worktree", async () => {
+    const { result } = renderHook(() => useSessionFiles("FN-456", undefined, "done"));
+
+    await waitFor(() => expect(result.current.loading).toBe(false));
+
+    expect(result.current.files).toEqual([]);
+    expect(mockFetchSessionFiles).not.toHaveBeenCalled();
+  });
 });

packages/dashboard/app/hooks/useSessionFiles.ts

@@ -1,7 +1,7 @@
 import { useEffect, useState } from "react";
 import { fetchSessionFiles } from "../api";
 
-const ACTIVE_COLUMNS = new Set(["in-progress", "in-review"]);
+const ACTIVE_COLUMNS = new Set(["in-progress", "in-review", "done"]);
 
 interface UseSessionFilesResult {
   files: string[];