docs: rewrite ch15 observability + cross-refs for the live obs stack

ch15 is now an account of what's actually running, not a roadmap for
what we'd add: VictoriaMetrics + Jaeger + Grafana on 88oakappsUpdate
fronted by Cloudflare and bearer-gated nginx, vmagent in-cluster, the
internal/prom histogram set, the rollout's NetworkPolicy footprint,
the obs.88oakapps.com endpoint shape, the ~$0/700MB resource budget,
and a token-rotation runbook. The "what we still don't have" section
keeps log aggregation, alerting, and full distributed tracing as the
honest gap list.

Other touched docs:
- 00-overview: \"deliberately absent\" no longer claims we have no
  metrics — calls out the cross-cluster shape instead.
- 14-deployment-process: TL;DR now points at deploy-k3s/scripts/03-deploy.sh
  (full build + push + apply + obs vmagent), with the manual
  kubectl-set-image flow kept as the single-service path. Notes the
  IfNotPresent gotcha that bit us during the rollout.
- 16-failure-modes: adds vmagent-can't-reach-obs and Grafana-no-data.
- 18-cost: $0 line item for the obs stack on 88oakappsUpdate, with the
  CX32 migration trigger.
- 17/18 README + appendix b: link the new ch15, add the obs cheat
  sheet block.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/deployment/README.md

@@ -48,7 +48,7 @@ they do, and how to operate them.
 
 - [12 — Data Flow](./12-data-flow.md) — end-to-end request lifecycle
 - [14 — Deployment Process](./14-deployment-process.md) — how to roll new code
-- [15 — Observability](./15-observability.md) — logs, metrics, tracing
+- [15 — Observability](./15-observability.md) — VictoriaMetrics + Jaeger + Grafana on `obs.88oakapps.com`, vmagent in-cluster, Prometheus histograms in the Go API
 - [16 — Failure Modes](./16-failure-modes.md) — what happens when X dies
 - [17 — Runbook](./17-runbook.md) — common ops tasks