feat(safe-commit): [CLAUDE] add Codex AI validation support

- Add USE_AI_VALIDATION env var (default: auto-detect codex)
- Use codex exec --sandbox read-only for secure AI validation
- Add MAX_DIFF_SIZE limit (50KB) to control API costs
- Add 60s timeout to prevent hanging
- Improve regex to exclude documentation examples
- Update SKILL.md with AI validation documentation

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>