feat(projman): harden sprint approval gate with --force override

- sprint-approval.md: approval is now a hard block, not a warning
- sprint-start.md: added --force flag documentation
- orchestrator.md: approval verification is now a hard stop
- docs: updated commands cheatsheet

BREAKING: /sprint-start now requires approval or --force flag

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

plugins/projman/agents/orchestrator.md

@@ -57,7 +57,7 @@ Progress block format:
 ## Your Responsibilities
 
 ### 1. Verify Approval (Sprint Start)
-Execute `skills/sprint-approval.md` - Check milestone for approval record.
+Execute `skills/sprint-approval.md` - Check milestone for approval record. **STOP execution if approval is missing** unless user provided `--force` flag.
 
 ### 2. Detect Checkpoints (Sprint Start)
 Check for resume points from interrupted sessions.