Review Grafana: replace Helm upgrade plan with C2 Mikado chain (#258)

## Summary
- Delete the old 3-phase Helm chart upgrade plan (predates Mikado system)
- Create C2 Mikado chain with goal card `upgrade-grafana` and two leaf prereqs:
  - `kustomize-grafana-deployment` — convert Helm to kustomize manifests
  - `build-grafana-container` — home-built Grafana 12.x image (no upstream containers)
- Record first-ever Grafana review: currently at v11.4.0 on Helm chart 8.8.2
- Update service-versions.yaml, how-to index, and plans index

## Service Review Findings
- Grafana is healthy and synced in ArgoCD
- Running v11.4.0, latest upstream is 12.3.3
- Breaking changes for 12.x are low-risk (React panels only, UIDs compliant)
- PVC is disposable — dashboards and datasources are all config-provisioned

## Deployment and Testing
- [ ] No deployment needed — documentation-only change
- [ ] `docs-check-links` passes
- [ ] `docs-check-index` passes

Reviewed-on: https://forge.ops.eblu.me/eblume/blumeops/pulls/258

service-versions.yaml

@@ -82,10 +82,10 @@ services:
 
   - name: grafana
     type: argocd
-    last-reviewed: null
-    current-version: null
+    last-reviewed: 2026-02-23
+    current-version: "v11.4.0"
     upstream-source: https://github.com/grafana/grafana/releases
-    notes: Deployed via Helm chart
+    notes: Helm chart 8.8.2; Mikado chain to upgrade to 12.x with kustomize
 
   - name: cloudnative-pg
     type: argocd