Add GitHub Actions CI for release checks

[xichia]

Add a GitHub Actions workflow that runs the public framework checks on pull requests and pushes.

Acceptance criteria

• CI runs make all
• CI runs make validate-public
• CI runs make pre-release
• CI fails if generated files are stale
• CI does not require heavy dependencies
• README mentions the CI check once added

Suggested files

• .github/workflows/ci.yml
• README.md