ci: validate skill/agent frontmatter and links

## Track
DX — see ROADMAP

## Goal
GitHub Action that fails PRs when:
- agent/skill frontmatter missing required fields
- model pins are not `claude-opus-5` / `claude-sonnet-5` (unless allowlisted)
- broken relative links in SKILL.md / README

## Acceptance
- [ ] Workflow under `.github/workflows/`
- [ ] Script or action documented in CONTRIBUTING