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/components/__tests__/ChangedFilesModal.test.tsx

@@ -0,0 +1,102 @@
+import { describe, it, expect, vi, beforeEach, afterEach } from "vitest";
+import { render, screen, fireEvent } from "@testing-library/react";
+import { ChangedFilesModal } from "../ChangedFilesModal";
+import * as changedFilesHook from "../../hooks/useChangedFiles";
+
+vi.mock("../../hooks/useChangedFiles");
+
+const mockUseChangedFiles = vi.mocked(changedFilesHook.useChangedFiles);
+
+describe("ChangedFilesModal", () => {
+  const mockOnClose = vi.fn();
+  const mockSetSelectedFile = vi.fn();
+
+  beforeEach(() => {
+    vi.resetAllMocks();
+    mockUseChangedFiles.mockReturnValue({
+      files: [
+        { path: "src/a.ts", status: "modified", diff: "diff --git a/src/a.ts b/src/a.ts\n--- a/src/a.ts\n+++ b/src/a.ts\n+hello" },
+        { path: "src/b.ts", status: "added", diff: "diff --git a/src/b.ts b/src/b.ts" },
+      ],
+      loading: false,
+      error: null,
+      selectedFile: { path: "src/a.ts", status: "modified", diff: "diff --git a/src/a.ts b/src/a.ts\n--- a/src/a.ts\n+++ b/src/a.ts\n+hello" },
+      setSelectedFile: mockSetSelectedFile,
+    });
+  });
+
+  afterEach(() => {
+    vi.restoreAllMocks();
+  });
+
+  it("renders changed files and selected diff", () => {
+    render(
+      <ChangedFilesModal
+        taskId="KB-651"
+        worktree="/repo/.worktrees/kb-651"
+        column="in-progress"
+        isOpen={true}
+        onClose={mockOnClose}
+      />,
+    );
+
+    expect(screen.getByText("Changed Files — KB-651")).toBeInTheDocument();
+    expect(screen.getByText("src/a.ts")).toBeInTheDocument();
+    expect(screen.getByLabelText("Diff for src/a.ts")).toBeInTheDocument();
+    expect(screen.getByText(/\+hello/)).toBeInTheDocument();
+  });
+
+  it("allows selecting another file from the sidebar", () => {
+    render(
+      <ChangedFilesModal
+        taskId="KB-651"
+        worktree="/repo/.worktrees/kb-651"
+        column="in-progress"
+        isOpen={true}
+        onClose={mockOnClose}
+      />,
+    );
+
+    fireEvent.click(screen.getByRole("button", { name: /src\/b.ts/i }));
+
+    expect(mockSetSelectedFile).toHaveBeenCalledWith({ path: "src/b.ts", status: "added", diff: "diff --git a/src/b.ts b/src/b.ts" });
+  });
+
+  it("shows an empty state when there are no changed files", () => {
+    mockUseChangedFiles.mockReturnValue({
+      files: [],
+      loading: false,
+      error: null,
+      selectedFile: null,
+      setSelectedFile: mockSetSelectedFile,
+    });
+
+    render(
+      <ChangedFilesModal
+        taskId="KB-651"
+        worktree="/repo/.worktrees/kb-651"
+        column="in-progress"
+        isOpen={true}
+        onClose={mockOnClose}
+      />,
+    );
+
+    expect(screen.getByText("No files changed")).toBeInTheDocument();
+  });
+
+  it("closes on Escape", () => {
+    render(
+      <ChangedFilesModal
+        taskId="KB-651"
+        worktree="/repo/.worktrees/kb-651"
+        column="in-progress"
+        isOpen={true}
+        onClose={mockOnClose}
+      />,
+    );
+
+    fireEvent.keyDown(document, { key: "Escape" });
+
+    expect(mockOnClose).toHaveBeenCalledTimes(1);
+  });
+});