Remove nessus app and fix envoy-gateway OCI chart bug

- Delete nessus Application CRD, manifests, and bootstrap script
- Vendor envoy-gateway Helm chart (v1.6.3) locally to work around
  ArgoCD v3.3.3 Docker Hub OCI resolution bug
- Re-enable auto-sync for envoy-gateway

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

charts/envoy-gateway/templates/namespace.yaml

@@ -0,0 +1,6 @@
+{{ if .Values.createNamespace }}
+apiVersion: v1
+kind: Namespace
+metadata:
+  name: '{{ .Release.Namespace }}' 
+{{ end }}