Decouple mpc-ceremony release validation from Relay

[mellowcroc]

Summary

• remove the pinned Relay commit and Relay checkout from mpc-ceremony release validation
• keep proof-tool release reproducibility and CLI validation independently owned
• document that exact released binary pairs are tested by the downstream ceremony-kit gate

This supersedes #11: the production-size Relay compatibility job is moved out of proof-tool release validation rather than given a larger timeout.

Validation

• go test ./cmd/mpc-ceremony
• Actionlint
• no Relay commit or repository checkout remains in proof-tool workflows or release documentation