Add detailed API reference and update rollout playbook:
**New: docs/api/SCHEDULER_HEALTH.md**
- Complete endpoint reference for /api/monitoring/scheduler/health
- Request/response structure with field descriptions
- Enhanced "instances" array documentation
- Example responses showing all states (healthy, transient, DLQ)
- Useful jq queries for troubleshooting:
- Find instances with errors
- List DLQ entries
- Show open circuit breakers
- Sort by failure streaks
- Migration guide (legacy → new fields)
- Troubleshooting examples with real scenarios
**Updated: docs/operations/ADAPTIVE_POLLING_ROLLOUT.md**
- Enhanced "Accessing Scheduler Health API" section (§6)
- Added examples using new instances[] array
- Updated queries to use pollStatus, breaker, deadLetter fields
- Practical jq commands for operators
**Key Documentation Features:**
- Complete JSON schema with examples
- All new fields documented with types and descriptions
- Real-world troubleshooting scenarios
- Copy-paste ready jq queries
- Migration path for existing integrations
- Backward compatibility notes
Operators can now:
- Find error messages without log digging
- Understand circuit breaker states
- Track DLQ entries with full context
- Diagnose issues using single API call
Part of Phase 2 follow-up - enhanced observability