feat: implement reference-cache for faster checkouts

- Add `reference-cache` input to action.yml
- Introduce `GitCacheHelper` for bare clone cache management
- Prevent race conditions with `proper-lockfile` and atomic directory renames
- Support iterative submodule caching and robust relative URL resolution
- Append to `info/alternates` preserving existing alternate references
- Add fallback to standard clone on submodule cache failure
- Add unit tests for `GitCacheHelper`

Signed-off-by: Michael Wyraz <mw@brick4u.de>

package.json

@@ -38,6 +38,7 @@
   "devDependencies": {
     "@types/jest": "^29.5.12",
     "@types/node": "^24.1.0",
+    "@types/proper-lockfile": "^4.1.4",
     "@types/uuid": "^9.0.8",
     "@typescript-eslint/eslint-plugin": "^7.9.0",
     "@typescript-eslint/parser": "^7.9.0",
@@ -49,6 +50,7 @@
     "jest-circus": "^29.7.0",
     "js-yaml": "^4.1.0",
     "prettier": "^3.3.3",
+    "proper-lockfile": "^4.1.2",
     "ts-jest": "^29.2.5",
     "typescript": "^5.5.4"
   }