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/applicationsets/kustomization.yaml

@@ -3,5 +3,6 @@ apiVersion: kustomize.config.k8s.io/v1beta1
 kind: Kustomization
 
 resources:
+  - observability.yaml
   - platform.yaml
   - storage.yaml

argocd/applicationsets/observability.yaml

@@ -0,0 +1,31 @@
+---
+apiVersion: argoproj.io/v1alpha1
+kind: ApplicationSet
+metadata:
+  name: observability-apps
+  namespace: argocd
+spec:
+  generators:
+  - git:
+      repoURL: https://git.unkin.net/unkin/argocd-apps
+      revision: HEAD
+      directories:
+      - path: apps/overlays/*/observability
+  template:
+    metadata:
+      name: 'observability-{{path[3]}}'
+    spec:
+      project: observability
+      source:
+        repoURL: https://git.unkin.net/unkin/argocd-apps
+        targetRevision: HEAD
+        path: '{{path}}'
+      destination:
+        server: https://kubernetes.default.svc
+        namespace: '{{path[3]}}'
+      syncPolicy:
+        automated:
+          prune: true
+          selfHeal: true
+        syncOptions:
+        - ServerSideApply=true