GitHub Actions reference pack

Goal

Provide a canonical GitHub Actions flow for first-proof adoption.

Required workflow steps

1. Checkout source.
2. Setup Python (3.11/3.12/3.13 matrix).
3. Install test/runtime dependencies.
4. Run make first-proof-verify.
5. Upload build/first-proof/* artifacts.

Suggested follow-up lanes

• make owner-escalation-payload
• make platform-readiness-dependency-radar