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-plan.md

@@ -18,6 +18,7 @@ agent: planner
 - skills/sprint-approval.md
 - skills/planning-workflow.md
 - skills/label-taxonomy/labels-reference.md
+- skills/sprint-lifecycle.md
 
 ## Purpose
 
@@ -35,6 +36,7 @@ Provide sprint goals as natural language input, or prepare input via:
 Execute the planning workflow as defined in `skills/planning-workflow.md`.
 
 **Key steps:**
+0. **Check Lifecycle State** - Execute `skills/sprint-lifecycle.md` check protocol. Expect idle state. Set `Sprint/Planning` after planning completes. Warn and stop if sprint is in another active state (unless `--force`).
 1. Run pre-planning validations (branch, repo org, labels)
 2. Detect input source (file, wiki, or conversation)
 3. Search relevant lessons learned