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/18-cost.md

@@ -58,6 +58,20 @@ honeyDue.
 |---|---:|
 | Gitea container registry | **$0** |
 
+### Observability (88oakappsUpdate)
+
+VictoriaMetrics + Jaeger + Grafana co-tenant on the existing Linode
+VPS that hosts PostHog. ~700 MB RAM, 21 GB disk — fits inside the
+existing instance. Not charged to honeyDue.
+
+| Item | Monthly |
+|---|---:|
+| Self-hosted obs stack on `88oakappsUpdate` | **$0** |
+
+Migration trigger: when the obs stack starts pressuring PostHog or
+needs hard isolation, move to a dedicated Hetzner CX32 (~$8/mo).
+See [Chapter 15 — When to move off](./15-observability.md).
+
 ### Total infrastructure
 
 | Category | Monthly |
@@ -67,6 +81,7 @@ honeyDue.
 | Storage | ~$0.30 |
 | Edge | $0 |
 | Registry | $0 |
+| Observability | $0 |
 | **Total** | **~$30** |
 
 ## External SaaS