fix: address all review findings on PR #1299
Remaining items from adversarial review:
- Add type: prompt to command frontmatter (consistency with complete-milestone)
- Replace git stats step with 4-method fallback chain (tag → STATE.md date →
earliest phase commit → skip gracefully)
- Add 7 fixture-based behavioral tests: archived milestone discovery,
multi-phase artifact discovery with varying completeness, empty .planning/
handling, output filename validation, git stats fallback verification,
type: prompt frontmatter check
Tests: 25/25 milestone-summary, 1236/1236 full suite
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
e4ca76dbb7