Add architecture doc for master-broken incident system
Context
The master-broken incident flow in triage-ops spans multiple files across 3 directories. Understanding the end-to-end flow requires reading ~10 source files. This has been re-explored multiple times.
What's in this MR?
Adds triage/triage/pipeline_failure/ARCHITECTURE.md — placed next to the code it documents. Covers:
- End-to-end flow from webhook to escalation
- All 8 config classes with matching rules
- Auto-triage logic (~7 root cause patterns)
- Duplicate detection mechanism
- Slack notification routing
- Escalation SLA timeline (10min, 30min, 3h40, 4h)
- Related projects (Observer, ci-alerts, EP-infra)