refactor: restructure skill markdown URLs and improve the site

[raymondk]

• return markdown urls from .well-known
• add note about not relying on cache
• fix the text on how it works
• add the source commit for a skill
• add times and commit to skill

[marc0olo]

Looking good. Two remaining things:

• api/skills.json.ts and api/skills/[slug].json.ts still use githubUrl() (main branch) for urls.source. Now that the skill page shows a commit-pinned link, the JSON API is out of sync — updated date and source URL describe different versions. Worth switching to githubCommitUrl (needs getSkillGitInfo in those two files).

• Nit: the new .ic-assets.json5 entry is commented "User discovery endpoints" — the adjacent rule says "Agent discovery endpoints". Suggest something like "Raw skill markdown endpoints" to match what it actually is.

[github-actions]

Skill Validation Report

No skill files were changed in this PR — validation skipped.

[marc0olo]

Thanks — api/skills/[slug].json.ts looks good now. api/skills.json.ts (the full index) still uses githubUrl() for urls.source. Since getSkillUpdatedAt already calls getSkillGitInfo internally, the SHA is already being computed and discarded — a small refactor to destructure it would make the index consistent too. Low priority, but worth tracking.