feat: migrate observability to ArgoCD

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>

apps/base/observability/kustomization.yaml

@@ -0,0 +1,6 @@
+---
+apiVersion: kustomize.config.k8s.io/v1beta1
+kind: Kustomization
+
+resources:
+  - namespace.yaml

apps/base/observability/namespace.yaml

@@ -0,0 +1,7 @@
+---
+apiVersion: v1
+kind: Namespace
+metadata:
+  labels:
+    app.kubernetes.io/name: observability
+  name: observability