feat: add fact-check and seo-review skills for content quality

- Add fact-check skill for verifying technical accuracy of concept pages
- Add seo-review skill for SEO audits with 27-point scoring system
- Move skills to both .claude/skills and .opencode/skill for compatibility
- Rename opencode.json to opencode.jsonc with skill configuration
- Update .gitignore to track .opencode/skill while ignoring local files
- Update CLAUDE.md with documentation for all three skills