Harden MCP registry publishing

[thymikee]

Summary

Harden the MCP registry publication workflow so the publisher tool is acquired deterministically and verified before release authentication or publication can occur.

Security-sensitive findings and reproduction details are intentionally omitted from this public issue. The implementation PR will carry the reviewed fix and regression evidence.

Scope

• Make publisher acquisition deterministic and integrity-checked.
• Keep verification ahead of release authentication and publishing.
• Add a structural regression for the workflow ordering.
• Document the controlled dependency-update procedure for maintainers.

Acceptance criteria

• The workflow cannot execute an unverified publisher artifact.
• Release authentication is unavailable until all publisher checks succeed.
• The pinned publisher identity and version are checked by CI.
• Updating the publisher requires an explicit, reviewable integrity update.