feat(ci): pivot to ArgoCD GitOps, fix Traefik ingress after real-cluster check

Direct SSH access to the actual clusters (node1-3 "prod" HA, dev-k8s-1-3
"dev") revealed two things that made the previous design wrong:

1. Neither cluster has vncmail/vnc-ca namespaces or a bulwark ingress at
   all - the "live sandbox" referenced in this repo's docs/manifests was
   never actually applied anywhere. Both ingress.yaml's ingressClassName
   (public) and cert-manager issuer (letsencrypt-prod) were also wrong:
   both clusters run Traefik (class is literally named `traefik`), and
   only dev-k8s has any ClusterIssuer at all (`letsencrypt-staging`).
   node1-3 has zero ClusterIssuers configured.

2. dev-k8s already has ArgoCD installed, idle, zero Applications - more
   idiomatic to use it than have GitLab Runner execute kubectl directly.

Pivots .gitlab-ci.yml: build+push image, then commit the tag into a small
per-overlay Component (overlays/{dev,prod}/image-tag/) that ArgoCD's
Application watches - CI never touches the cluster, only the registry and
this repo. dev's Application (vncmail-dev) is registered and applied
already (manual sync for now, until the one-time namespace secret
bootstrap is done - see VNCMAIL-SETUP.md). prod's Application is
scaffolded in deploy/argocd/ but deliberately not applied - it targets a
different cluster (node1-3) that isn't registered with ArgoCD yet, and
there's still no real prod hostname/Stalwart/ClusterIssuer.

Fixes base/ingress.yaml to the real ingressClassName: traefik (was the
nginx-style `public`, which doesn't exist on either cluster) and gives
each overlay its own cert-manager issuer patch instead of one hardcoded
value, since dev and prod need different (or, for prod, nonexistent)
issuers.

deploy/k8s/overlays/prod/patch-ingress.yaml

@@ -3,6 +3,12 @@
 # deliberately unresolvable: applying this overlay as committed will not
 # issue a cert or route traffic anywhere. Replace both occurrences below,
 # and the matching TLS secretName, before Phase D (first real prod deploy).
+#
+# Targets node1-3 (the HA "prod" cluster). Deliberately does NOT override
+# base's `cert-manager.io/cluster-issuer: CHANGEME` — node1-3 has ZERO
+# ClusterIssuers configured today (confirmed via direct access). A human
+# needs to create a real one there (ACME account, DNS-01 or HTTP-01 solver)
+# before this can be anything but a placeholder.
 apiVersion: networking.k8s.io/v1
 kind: Ingress
 metadata: