docs(spec): add Phase 10 implementation plan for Homebrew tap rename

Changes:
- Created spec/milestone-12-renaming-to-anvs/phase-10.md with comprehensive plan
- Covers Homebrew tap repository rename (homebrew-xvn → homebrew-anvs)
- Includes formula file rename and updates (xvn.rb → anvs.rb)
- 16 detailed tasks with specific commands and expected outputs
- 115 action checklist items for tracking progress
- Verification checklist, success criteria, rollback plan
- Common issues and solutions section
- Homebrew tap naming convention reference
- Estimated 30-45 minutes + post-release testing time

Related to Milestone 12 (xvn → anvs rename) Phase 10

Author: cameronolivier