Milestone: open/freeze/undergoing/completed/closed (was open/pending/deferred/progressing/closed)
Task: open/pending/undergoing/completed/closed (was open/pending/progressing/closed)
- Add MilestoneStatusEnum to schemas with typed validation
- Add started_at field to Milestone model
- Update all router/CLI references from progressing->undergoing
- Add completed status handling in task transition logic