feat(argocd): deploy SOPS CMP plugin for secret decryption — Stage 0 grafana

Adds ConfigManagementPlugin (CMP) sidecar to argocd-repoServer. Plugin decrypts
*.enc.yaml files with age key from sops-age Secret, emits plain Kubernetes Secrets.

Stage 0: grafana only (2 Secrets: grafana-oidc + new grafana-admin). Updates
grafana-values.yaml to wire admin.existingSecret (chart-native support).

CMP Application (00-secrets.yaml) syncs at wave 0 before grafana/loki/authentik.
Decryption happens on-demand during sync, no pre-built Secret commits. Stages 1-4
(loki/authentik/forgejo/temporal) extend plugin script incrementally after
verification.

Co-Authored-By: Claude Haiku 4.5 <[email protected]>

k8s/argocd/apps/00-secrets.yaml

@@ -0,0 +1,26 @@
+apiVersion: argoproj.io/v1alpha1
+kind: Application
+metadata:
+  name: sops-secrets
+  namespace: argocd
+  finalizers:
+    - resources-finalizer.argocd.argoproj.io
+  annotations:
+    argocd.argoproj.io/sync-wave: "0"
+spec:
+  project: homelab
+  revisionHistoryLimit: 3
+  syncPolicy:
+    automated:
+      prune: true
+      selfHeal: true
+  syncOptions:
+    - CreateNamespace=true
+  source:
+    repoURL: http://forgejo.riotpiao.com:3000/riotpiao.com/homelab.git
+    targetRevision: main
+    path: k8s/security/sops-secrets
+    plugin: {}
+  destination:
+    server: https://kubernetes.default.svc
+  revisionHistoryLimit: 3