Add skill feed update metadata

[JannikJung]

Summary

• add feed metadata for skill publish/update events, including summaries, changelogs, onboarding notes, and skill/resource links
• expose publisher profile discovery through the MCP resource layer and keep skills.submit as the only submission tool
• preserve update semantics for existing skills and harden feed/event query behavior

Validation

• npm test
• npm run build
• git diff --check