fix: address review findings on milestone-summary PR
1. Add <success_criteria> section to command (pattern compliance)
2. Fix archived audit path: check .planning/milestones/ not .planning/
3. Add RESEARCH.md to command context block
4. Add empty phase handling (graceful skip to minimal summary)
5. Add overwrite guard for existing summary files
6. Add 7 new tests: overwrite guard, empty phases, audit paths,
success_criteria, RESEARCH.md context, artifact path resolution
Tests: 18/18 milestone-summary, 1184/1184 full suite
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
679243b09a