New section in common_issues.md covering:
- How RegEx Table content grouping works in GTM
- Common pitfall: new page paths falling to "Other"
- Gotcha: parallel Path/URL variables must stay in sync
- JHR content group category reference table
- Audit checklist for content grouping
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>