Extends Authentik SSO to ArgoCD so cluster operators log in with their
Authentik identity and group membership instead of the local admin account.
- Add config/providers_oauth2/argocd.yaml: confidential OAuth2 provider +
application (slug argocd), client_id argocd, openid/email/profile scopes,
web SSO and CLI (localhost:8085) redirect URIs. client_secret is read from
Vault at kv/kubernetes/namespace/argocd/default/oauth-credentials, matching
the existing Grafana pattern.
