Home-build grafana-sidecar container (#281)

## Summary
- Home-build the k8s-sidecar container (`grafana-sidecar`) from forge mirror, replacing upstream `quay.io/kiwigrid/k8s-sidecar:1.28.0`
- Pinned to v1.28.0 — v2.x deferred due to 135% memory regression and readOnlyRootFilesystem crashloop
- Adds Dockerfile, service-versions entry, docs, and changelog fragment
- Manifest switch to home-built image pending container build

## Deployment and Testing
- [ ] `mise run container-build-and-release grafana-sidecar`
- [ ] Update kustomization.yaml with built image tag
- [ ] `argocd app set grafana --revision feature/grafana-sidecar && argocd app sync grafana`
- [ ] Verify sidecar logs and dashboards at https://grafana.ops.eblu.me
- [ ] Post-merge: `argocd app set grafana --revision main && argocd app sync grafana`

Reviewed-on: #281

service-versions.yaml

@@ -87,6 +87,13 @@ services:
     upstream-source: https://github.com/grafana/grafana/releases
     notes: Home-built container; upgrading to 12.x via Mikado chain
 
+  - name: grafana-sidecar
+    type: argocd
+    last-reviewed: "2026-03-03"
+    current-version: "1.28.0"
+    upstream-source: https://github.com/kiwigrid/k8s-sidecar/releases
+    notes: Dashboard ConfigMap watcher sidecar in grafana deployment
+
   - name: cloudnative-pg
     type: argocd
     last-reviewed: 2026-02-24