GitHub Action: CI sync + validate .skillr/ in PRs

[eooo-io]

From FEATURE_IDEAS.md #12. Already tracked as "post-v1.0" in PLAN.md; filing for visibility.

A reusable GitHub Action that:

1. On push to main: runs `skillr sync` and commits any changes to provider configs (so `.cursor/rules/` etc. stay fresh automatically)
2. On PRs: validates skill frontmatter, runs `skillr lint`, fails the check on errors, and comments a summary

Acceptance

• Action published to the Marketplace
• Docs page under `docs/guide/github-action.md` with copy-paste workflow examples