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/projects/platform.yaml

@@ -10,6 +10,7 @@ spec:
     - https://git.unkin.net/unkin/argocd-apps
     - https://charts.jetstack.io
     - https://cloudnative-pg.github.io/charts
+    - https://helm.elastic.co
     - https://intel.github.io/helm-charts/
     - https://kubernetes-sigs.github.io/external-dns/
     - https://releases.rancher.com/server-charts/stable