docs: streamline README from 1241 to 293 lines

- Keep only v4.7.0 highlights (remove version history back to v3.6)
- Update slash commands to reflect hub-based structure
- Add Command Hubs table showing new organization
- Consolidate features into scannable sections
- Fix markdown linting issues

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

Author: silversurfer562