Reverts complex CMP plugin setup (helm chart doesn't support repoServer.extraContainers).
Instead: sops-secrets Application uses directory source (no plugin), emits placeholder
README. Manually-created Secrets (grafana-admin) live in target namespaces.
Full CMP plugin work deferred to future stage. Grafana values still wired to
admin.existingSecret (no-op until Secret exists, which it now does).
This unblocks cluster deployment without waiting for ArgoCD CMP plumbing.