Excellent editorial style guide updates! This PR adds valuable clarifications and improvements:

✅ Oxford comma guidelines - Clear guidance on when to use Oxford commas
✅ Em dash formatting - Proper spacing rules with examples
✅ Facilitator responsibilities - Important addition for group process mechanics
✅ Ampersand usage - Helpful guidelines for business names and abbreviations
✅ Parenthetical periods - Clear rules for period placement

All changes are well-documented with examples and external references. The additions enhance consistency and clarity for content creators. Ready to merge! 🚀