Implement Remote discovery and Fetch

[Traviinam]

Summary

• discover configured Remotes without exposing credential-bearing URLs
• add Repository and per-Remote Fetch operations with admission control
• reconcile Fetch results and publish per-Remote freshness
• expose Fetch through the protocol and Repository overview UI

Testing

• full suite: 34 files, 206 tests
• focused post-review suite: 5 files, 38 tests
• npm run build
• npm run lint
• npm run format:check

Closes #13