docs: canonicalize skill format guidance

[kamalsrini]

Summary

• Updates CONTRIBUTING.md to make SKILL.md the canonical skill entrypoint
• Removes stale skill.yaml + per-skill README.md primary-definition guidance
• Aligns new-skill instructions with README.md and SKILL_TEMPLATE.md

Linear

• UNI-5

Verification

• git diff --check

[github-actions]

Thanks for contributing! 🙏 To keep the queue reviewable, we allow one open PR per contributor at a time. You already have #11 open, so we're closing this one — please reopen it after that PR is resolved.

[kamalsrini]

Reopened — this was closed in error by the contribution gate, which should exempt maintainers. Fixing the gate so it skips collaborators reliably.

[github-actions]

Thanks for contributing! 🙏 To keep the queue reviewable, we allow one open PR per contributor at a time. You already have #11 open, so we're closing this one — please reopen it after that PR is resolved.