Add GCS cache backend with GitHub fallback

Port GCS support from danySam/gcs-cache. When gcs-bucket input is set,
cache to Google Cloud Storage via Application Default Credentials; else
fall back to GitHub's cache service transparently.

- New inputs: gcs-bucket, gcs-path-prefix (default: github-cache)
- New src/utils/gcsCache.ts wraps @actions/cache restore/save
- restoreImpl/saveImpl swap @actions/cache import for local wrapper
- Adds @google-cloud/storage ^7.16.0, bumps typescript to ^5.8.3
- Regenerated dist/ via npm run build

Skipped: upstream workflow removals, README/examples/RELEASES churn,
licensed/codeql config, GCS integration-test workflow.

save/action.yml

@@ -1,6 +1,6 @@
-name: 'Save a cache'
-description: 'Save Cache artifacts like dependencies and build outputs to improve workflow execution time'
-author: 'GitHub'
+name: 'Save Cache with GCS Support'
+description: 'Save cache artifacts to Google Cloud Storage or GitHub with automatic fallback'
+author: 'danySam'
 inputs:
   path:
     description: 'A list of files, directories, and wildcard patterns to cache'
@@ -15,8 +15,15 @@ inputs:
     description: 'An optional boolean when enabled, allows windows runners to save caches that can be restored on other platforms'
     default: 'false'
     required: false
+  gcs-bucket:
+    description: 'Google Cloud Storage bucket name to use for caching. When provided, GCS will be used as the cache backend.'
+    required: false
+  gcs-path-prefix:
+    description: 'Optional prefix path within the GCS bucket for cache files'
+    default: 'github-cache'
+    required: false
 runs:
-  using: 'node24'
+  using: 'node20'
   main: '../dist/save-only/index.js'
 branding:
   icon: 'archive'