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/skills/label-taxonomy/labels-reference.md

@@ -90,6 +90,18 @@ Repository-level labels are specific to each project.
 - `Tech/Vue` (#42b883) - Vue.js frontend framework
 - `Tech/FastAPI` (#009688) - FastAPI backend framework
 
+### Sprint Lifecycle (Milestone Metadata)
+
+These are tracked as milestone description metadata, not as Gitea issue labels. They are documented here for completeness.
+
+| Label | Description |
+|-------|-------------|
+| `Sprint/Planning` | Sprint planning in progress |
+| `Sprint/Executing` | Sprint execution in progress |
+| `Sprint/Reviewing` | Code review in progress |
+
+**Note:** Lifecycle state is stored in milestone description as `**Sprint State:** Sprint/Executing`. See `skills/sprint-lifecycle.md` for state machine rules.
+
 ### Domain (2 labels)
 
 Cross-plugin integration labels for domain-specific validation gates.