Add contributing guide and CI

[ramimbo]

Summary

• Add a concise CONTRIBUTING.md for project workflow
• Add GitHub Actions CI for syntax and Playwright fixture tests
• Document that CI runs the non-live checks on PRs and main

Testing

• node --check github-sortout.user.js
• npm test
• git diff --check

Live GitHub smoke tests remain manual because they depend on external GitHub page markup and network behavior.