refactor: move .opencode/skills to top-level skills/, remove skill-creator, add Skills to docs site

- Move .opencode/skills/harness-creator/ → skills/harness-creator/
- Remove skill-creator (third-party, now referenced via link to anthropics/skills)
- Add skills/ README.md (EN) and README-CN.md (ZH) with security audit notes
- Add VitePress skills pages (docs/en/skills/, docs/zh/skills/)
- Add Skills/技能 nav item to both EN and ZH site navigation
- Update root README.md and README-CN.md with quick start note and skills/ in repo tree

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: sanbuphy