chore: add project review agents and shared settings

- Add 4 review agents based on installed agent templates with
  Mantle-specific project context appended to each:
  technical-writer, product-manager, legal-compliance, reality-checker
- Add shared project settings (.claude/settings.json)
- Update .gitignore to commit .claude/settings.json and .claude/agents/
- Rename Product Owner to Product Manager in CLAUDE.md

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: michaelmcnees