Migrates legacy rule numbering (18, 20) to standardized sequence (53, 54) and adds new 18-preexisting-issues.md for handling pre-existing issues during development. This consolidates duplicate rule coverage across the old and new numbering schemes.
Files changed:
- Removed: 18-relative-units.md (moved to 53-relative-units.md)
- Removed: 20-testing-gate.md (moved to 54-testing-gate.md)
- Added: 18-preexisting-issues.md (new)
- Added: 53-relative-units.md (renamed from 18)
- Added: 54-testing-gate.md (renamed from 20)
