feat(hooks): add Unity Git Hooks integration and runHookGroups

Built-in support for Unity Git Hooks (com.frostebite.unitygithooks):
- Auto-detect UPM package in Packages/manifest.json
- Run init-unity-lefthook.js before hook installation
- Set CI-friendly env vars (disable background project mode)

New gitHooksRunBeforeBuild input runs specific lefthook groups before
the Unity build, allowing CI to trigger pre-commit or pre-push checks
that normally only fire on git events.

35 unit tests covering detection, init, CI env, group execution, and
failure handling.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

action.yml

@@ -345,6 +345,14 @@ inputs:
     default: ''
     description:
       'Comma-separated list of hook names to skip even when gitHooksEnabled is true. Example: pre-push,post-merge'
+  gitHooksRunBeforeBuild:
+    required: false
+    default: ''
+    description:
+      'Comma-separated list of lefthook hook groups to run before the Unity build. Allows CI to
+      trigger checks that normally only run on git events. Example: pre-commit,pre-push. Requires
+      lefthook. Works with Unity Git Hooks (com.frostebite.unitygithooks) when installed as a UPM
+      package — the init script runs automatically.'
   providerExecutable:
     required: false
     default: ''