Adds test file with validation scenarios:
- Schema validation for required/optional params
- All 8 state labels are supported
- Same-state transitions handled gracefully
- Audit logging verification
Includes manual test scenarios for integration verification.
Addresses issue #71 acceptance criteria:
✅ Can transition any issue to any valid state
✅ Logs reason to audit trail
✅ Updates GitHub label (projects.json managed by session_health)
✅ Documented as orchestrator-only in AGENTS.md
