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/commands/sprint-close.md

@@ -13,6 +13,7 @@ agent: orchestrator
 - skills/rfc-workflow.md
 - skills/progress-tracking.md
 - skills/git-workflow.md
+- skills/sprint-lifecycle.md
 
 ## Purpose
 
@@ -26,6 +27,7 @@ Run `/sprint-close` when sprint work is complete.
 
 Execute the sprint close workflow:
 
+0. **Check Lifecycle State** - Execute `skills/sprint-lifecycle.md` check protocol. Expect `Sprint/Reviewing`. Clear all Sprint/* labels (return to idle) at the END of close workflow, after all other steps. Warn if in wrong state (allow with `--force`).
 1. **Review Sprint Completion** - Verify issues closed or moved to backlog
 2. **Capture Lessons Learned** - Interview user about challenges and insights
 3. **Tag for Discoverability** - Apply technology, component, and pattern tags