fix: validate skill YAML and release 3.5.1 - #52

Summary

• parse every Claude and Codex SKILL.md front matter as real YAML instead of checking fields with regex
• require non-empty string name and description values across skills/ and .agents/skills/
• document the release gate and root-only gray-matter dependency
• bump CLI, workspace, plugin manifests, marketplace, and lockfiles to 3.5.1

Verification

• node scripts/check-codex-surface.mjs
• malformed YAML regression fixture rejected
• npm run lint
• npm run typecheck
• npm test — 46 tests passed
• npm run check:prompts
• npm run validate:dogfood
• Codex prompt-input skill loading smoke test passed without YAML warnings