Add optional phase archival to milestone completion and a standalone
/gsd:cleanup command for retroactive use. Phase dirs move to
.planning/milestones/v{X.Y}-phases/, reducing phases/ clutter after
multiple milestones.
Core changes:
- getArchivedPhaseDirs() and searchPhaseInDir() helpers in gsd-tools
- findPhaseInternal() searches archives when phase not found in current
- cmdPhasesList() accepts --include-archived flag
- cmdHistoryDigest() scans both current and archived phases
- cmdMilestoneComplete() accepts --archive-phases flag
- Workflow globs replaced with find-phase/phases-list CLI calls
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>