Add scheduled workflow that validates community Unity packages compile
and build correctly using unity-builder. Runs weekly on Sunday.
Includes:
- YAML plugin registry (community-plugins.yml) for package listings
- Matrix expansion across plugins and platforms
- Automatic failure reporting via GitHub issues
- Manual trigger with plugin filter and Unity version override
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
