Upgrade grafana-sidecar 1.28.0 → 2.6.0 + container.py port (#332)

## Summary

- Upgrade grafana-sidecar from 1.28.0 to 2.6.0 (the 2.x memory regression #462 is resolved; ~35MB static overhead is acceptable)
- Port build from Dockerfile to native Dagger container.py
- Add liveness/readiness probes using the new /healthz endpoint on port 8080
- Update docs to reflect container.py migration and remove stale pin note

## Test plan

- [ ] Build container: `mise run container-build-and-release grafana-sidecar`
- [ ] Update kustomization tag with new image tag
- [ ] Deploy from branch: `argocd app set grafana --revision grafana-sidecar-2.6.0 && argocd app sync grafana`
- [ ] Verify sidecar health endpoint: `kubectl exec -n monitoring <pod> -c grafana-sc-dashboard -- wget -qO- http://localhost:8080/healthz`
- [ ] Verify dashboards load in Grafana UI
- [ ] `mise run services-check`

Reviewed-on: #332

service-versions.yaml

@@ -106,8 +106,8 @@ services:
   - name: grafana-sidecar
     type: argocd
     parent: grafana
-    last-reviewed: "2026-03-03"
-    current-version: "1.28.0"
+    last-reviewed: "2026-04-13"
+    current-version: "2.6.0"
     upstream-source: https://github.com/kiwigrid/k8s-sidecar/releases
     notes: Dashboard ConfigMap watcher sidecar in grafana deployment