feat: migrate cattle-system (Rancher) from Terraform to ArgoCD (#39)

- Add cattle-system base ArgoCD application with namespace, Vault integration, and ingress
- Create cattle-system overlay for au-syd1 with Rancher Helm chart configuration
- Update platform ApplicationSet to include cattle-system deployment
- Update platform project to include Rancher Helm repository as source
- Configure Rancher v2.13.1 with HA, TLS, audit logging, and bootstrap secret from Vault
- Maintain one-to-one migration from Terraform configuration

Reviewed-on: #39

apps/overlays/au-syd1/cattle-system/values.yaml

@@ -0,0 +1,40 @@
+hostname: rancher.k8s.syd1.au.unkin.net
+
+bootstrapPassword: ""
+
+extraEnv:
+  - name: CATTLE_BOOTSTRAP_PASSWORD
+    valueFrom:
+      secretKeyRef:
+        name: rancher-bootstrap-secret
+        key: password
+
+ingress:
+  enabled: false
+
+tls: external
+
+replicas: 3
+
+priorityClassName: rancher-critical
+
+# Resource limits for production
+resources:
+  limits:
+    cpu: 2000m
+    memory: 8Gi
+  requests:
+    cpu: 50m
+    memory: 256Mi
+
+# Additional security settings
+antiAffinity: preferred
+
+# Audit logging configuration
+auditLog:
+  enabled: true
+  destination: sidecar
+  level: 1
+  maxAge: 7
+  maxBackup: 3
+  maxSize: 100