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-start.md

@@ -15,6 +15,7 @@ agent: orchestrator
 - skills/git-workflow.md
 - skills/progress-tracking.md
 - skills/runaway-detection.md
+- skills/sprint-lifecycle.md
 
 ## Purpose
 
@@ -31,7 +32,7 @@ Run `/sprint-start` when ready to begin executing a planned sprint.
 
 Execute the sprint start workflow:
 
-1. **Verify Sprint Approval** (required) - Check milestone for approval record. STOP if missing unless `--force` flag provided.
+1. **Verify Sprint Approval & Lifecycle State** (required) - Check milestone for approval record. STOP if missing unless `--force` flag provided. Also verify lifecycle state is `Sprint/Planning` per `skills/sprint-lifecycle.md`. Set `Sprint/Executing` after verification passes.
 2. **Detect Checkpoints** - Check for resume points from interrupted sessions
 3. **Fetch Sprint Issues** - Get open issues from milestone
 4. **Analyze Dependencies** - Use `get_execution_order` for parallel batches