feat(KB-651): add changed files diff viewer for tasks

- Add backend API endpoint to get changed files and diffs for a task worktree
- Add useChangedFiles hook for fetching and managing diff data
- Create ChangedFilesModal component with file list and diff viewer
- Integrate modal into TaskCard with click handler to view changes
- Add frontend API integration and type definitions
- Include unit tests for hook and modal components
- Add changeset for the new feature
- Remove deprecated mission-related test files

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

@@ -0,0 +1,79 @@
+import { describe, it, expect, vi, beforeEach, afterEach } from "vitest";
+import { renderHook, waitFor, act } from "@testing-library/react";
+import { useChangedFiles } from "../useChangedFiles";
+import * as api from "../../api";
+
+vi.mock("../../api", () => ({
+  fetchTaskFileDiffs: vi.fn(),
+}));
+
+const mockFetchTaskFileDiffs = vi.mocked(api.fetchTaskFileDiffs);
+
+describe("useChangedFiles", () => {
+  beforeEach(() => {
+    mockFetchTaskFileDiffs.mockReset();
+  });
+
+  afterEach(() => {
+    vi.clearAllMocks();
+  });
+
+  it("fetches changed files for active tasks with a worktree and auto-selects first file", async () => {
+    mockFetchTaskFileDiffs.mockResolvedValueOnce([
+      { path: "src/a.ts", status: "modified", diff: "diff --git a/src/a.ts b/src/a.ts" },
+      { path: "src/b.ts", status: "added", diff: "diff --git a/src/b.ts b/src/b.ts" },
+    ]);
+
+    const { result } = renderHook(() => useChangedFiles("KB-651", "/repo/.worktrees/kb-651", "in-progress"));
+
+    await waitFor(() => expect(result.current.loading).toBe(false));
+
+    expect(result.current.error).toBeNull();
+    expect(result.current.files).toHaveLength(2);
+    expect(result.current.selectedFile?.path).toBe("src/a.ts");
+    expect(mockFetchTaskFileDiffs).toHaveBeenCalledWith("KB-651");
+  });
+
+  it("does not fetch for tasks without worktrees or inactive columns", async () => {
+    const { result: noWorktree } = renderHook(() => useChangedFiles("KB-651", undefined, "in-progress"));
+    const { result: inactive } = renderHook(() => useChangedFiles("KB-651", "/repo/.worktrees/kb-651", "todo"));
+
+    await waitFor(() => expect(noWorktree.current.loading).toBe(false));
+    await waitFor(() => expect(inactive.current.loading).toBe(false));
+
+    expect(noWorktree.current.files).toEqual([]);
+    expect(noWorktree.current.selectedFile).toBeNull();
+    expect(inactive.current.files).toEqual([]);
+    expect(inactive.current.selectedFile).toBeNull();
+    expect(mockFetchTaskFileDiffs).not.toHaveBeenCalled();
+  });
+
+  it("returns an error state on fetch failure", async () => {
+    mockFetchTaskFileDiffs.mockRejectedValueOnce(new Error("boom"));
+
+    const { result } = renderHook(() => useChangedFiles("KB-651", "/repo/.worktrees/kb-651", "in-review"));
+
+    await waitFor(() => expect(result.current.loading).toBe(false));
+
+    expect(result.current.files).toEqual([]);
+    expect(result.current.selectedFile).toBeNull();
+    expect(result.current.error).toBe("boom");
+  });
+
+  it("allows selecting a different file after data loads", async () => {
+    mockFetchTaskFileDiffs.mockResolvedValueOnce([
+      { path: "src/a.ts", status: "modified", diff: "first" },
+      { path: "src/b.ts", status: "added", diff: "second" },
+    ]);
+
+    const { result } = renderHook(() => useChangedFiles("KB-651", "/repo/.worktrees/kb-651", "in-progress"));
+
+    await waitFor(() => expect(result.current.loading).toBe(false));
+
+    act(() => {
+      result.current.setSelectedFile(result.current.files[1]!);
+    });
+
+    expect(result.current.selectedFile?.path).toBe("src/b.ts");
+  });
+});