feat: migrate observability to ArgoCD (#82)

Migrate Victoria Metrics cluster and agent from Terragrunt to ArgoCD/Kustomize.
Creates new observability AppProject and ApplicationSet.
Deploys victoria-metrics-cluster v0.33.0 (vmselect/vminsert/vmstorage with
HPA, PDB, ingress) and victoria-metrics-agent v0.30.0 (3 replicas, k8s scrape
configs) in the observability namespace.

💘 Generated with Crush

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

Reviewed-on: #82

argocd/projects/observability.yaml

@@ -0,0 +1,24 @@
+---
+apiVersion: argoproj.io/v1alpha1
+kind: AppProject
+metadata:
+  name: observability
+  namespace: argocd
+spec:
+  description: Observability stack (metrics, monitoring)
+  sourceRepos:
+    - https://git.unkin.net/unkin/argocd-apps
+    - https://victoriametrics.github.io/helm-charts/
+  destinations:
+    - namespace: 'observability'
+      server: https://kubernetes.default.svc
+  clusterResourceWhitelist:
+    - group: ''
+      kind: Namespace
+    - group: 'rbac.authorization.k8s.io'
+      kind: ClusterRole
+    - group: 'rbac.authorization.k8s.io'
+      kind: ClusterRoleBinding
+  namespaceResourceWhitelist:
+    - group: '*'
+      kind: '*'