feat(KB-093): add PR-first merge mode with configurable merge strategies

- Add mergeStrategy setting (fast-forward, squash, merge-commit) to config
- Implement PR-first auto-completion flow that monitors PR merge status
- Wire PR monitoring service to detect merge completion and trigger auto-close
- Add PR status UI to dashboard with merge progress indicator
- Update settings modal with merge strategy selector
- Add changeset for PR-first merge mode feature

packages/cli/src/commands/__tests__/dashboard.test.ts

@@ -52,8 +52,16 @@ const mockListen = vi.fn((port: number) => {
   return server;
 });
 
+const MockGitHubClient = vi.fn().mockImplementation(() => ({
+  findPrForBranch: vi.fn(),
+  createPr: vi.fn(),
+  getPrMergeStatus: vi.fn(),
+  mergePr: vi.fn(),
+}));
+
 vi.mock("@kb/dashboard", () => ({
   createServer: vi.fn(() => ({ listen: mockListen })),
+  GitHubClient: MockGitHubClient,
 }));
 
 // ── Mock @kb/engine ────────────────────────────────────────────────