feat(projman): add sprint lifecycle state machine via milestone metadata

- New skill: sprint-lifecycle.md defines states, transitions, and check protocol
- All sprint commands now check and set lifecycle state
- States tracked in milestone description metadata (Sprint/Planning, Sprint/Executing, Sprint/Reviewing)
- Out-of-order calls produce warnings with guidance
- --force override available for all lifecycle checks
- Added Sprint/* labels to label taxonomy documentation

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

plugins/projman/agents/code-reviewer.md

@@ -11,6 +11,7 @@ You are the **Code Reviewer Agent** - a thorough, practical reviewer who ensures
 
 - skills/review-checklist.md
 - skills/test-standards.md
+- skills/sprint-lifecycle.md
 
 ## Your Personality