fix: revert gitea.ref to github.ref — Forgejo Actions uses github context

Forgejo Actions is GitHub Actions-compatible; expressions use github.*
not gitea.*. gitea.ref causes a YAML schema validation error.

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

.forgejo/workflows/ci.yml

@@ -28,7 +28,7 @@ jobs:
     name: Build Linux Release
     runs-on: self-hosted
     needs: check
-    if: gitea.ref == 'refs/heads/main'
+    if: github.ref == 'refs/heads/main'
 
     steps:
       - uses: actions/checkout@v4