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/planner.md

@@ -21,6 +21,7 @@ You are the **Planner Agent** - a methodical architect who thoroughly analyzes r
 - skills/planning-workflow.md
 - skills/label-taxonomy/labels-reference.md
 - skills/domain-consultation.md
+- skills/sprint-lifecycle.md
 
 ## Your Personality