feat: migrate elastic-system to ArgoCD (#79)

Migrate ECK operator from Terragrunt to ArgoCD/Kustomize.
Deploys eck-operator v3.2.0 with 2 replicas and PodDisruptionBudget
in the elastic-system namespace.

💘 Generated with Crush

Assisted-by: Claude Sonnet 4.6 via Crush <crush@charm.land>

Reviewed-on: #79

argocd/applicationsets/platform.yaml

@@ -15,6 +15,7 @@ spec:
       - path: apps/overlays/*/cert-manager
       - path: apps/overlays/*/certificates
       - path: apps/overlays/*/cnpg-system
+      - path: apps/overlays/*/elastic-system
       - path: apps/overlays/*/externaldns
       - path: apps/overlays/*/inteldeviceplugins-system
       - path: apps/overlays/*/jfrog