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-status.md

@@ -9,6 +9,7 @@ description: Check current sprint progress, identify blockers, optionally genera
 - skills/mcp-tools-reference.md
 - skills/progress-tracking.md
 - skills/dependency-management.md
+- skills/sprint-lifecycle.md
 
 ## Purpose
 
@@ -23,6 +24,7 @@ Check current sprint progress, identify blockers, and show execution status. Opt
 
 ## Workflow
 
+0. **Display Lifecycle State** - Read current Sprint/* state from milestone description per `skills/sprint-lifecycle.md` and display in output header.
 1. **Fetch Sprint Issues** - Get all issues for current milestone
 2. **Calculate Progress** - Count completed vs total issues
 3. **Identify Active Tasks** - Find issues with `Status/In-Progress`