cost/ — Phase 13c

Phase 13a does not ship cost manifests. Phase 13c (ch.13.10) adds:

• OpenCost install (pinned Helm, dedicated ns).
• Per-tenant cost label propagation (the ResourceQuota label flow already established in Part 08 ch.04, deepened).
• Grafana cost dashboards (per-tenant, per-region, per-workload-class).
• Budget alerts via Alertmanager.

Cross-ref Part 08 ch.04 (multi-tenancy + cost) and Part 10 ch.06 (cluster autoscaling cost) for the foundations.