feat(KB-030): add Git Manager dashboard feature

- Add GitManagerModal component with status, commits, branches, and worktrees tabs

- Implement git API endpoints for status, commits, diff, branches, worktrees, and actions

- Add git API client functions with error handling

- Integrate Git Manager into App and Header with toolbar button

- Add comprehensive tests for GitManagerModal component

- Include README documentation for dashboard package and changeset

packages/dashboard/package.json

@@ -36,6 +36,7 @@
   "devDependencies": {
     "@testing-library/jest-dom": "^6.9.1",
     "@testing-library/react": "^16.3.2",
+    "@testing-library/user-event": "^14.5.0",
     "@types/express": "^5.0.0",
     "@types/node": "^22.0.0",
     "@types/react": "^19.0.0",