feat: remove ArgoCD Image Updater, replaced by Forgejo webhook

Image promotion now handled by api-gw webhook endpoint
(/v1/webhooks/forgejo) which patches ArgoCD Application CRs
on Forgejo package push events (homelab-frontend PR #55).

- Delete Image Updater Application and values
- Remove argocd-image-updater.argoproj.io annotations from all apps
- Registry secrets kept (used by webhook digest queries + imagePullSecrets)

k8s/argocd/apps/71-poimen-toolkit.yaml

@@ -5,11 +5,9 @@ metadata:
   namespace: argocd
   annotations:
     argocd.argoproj.io/sync-wave: "7"
-    argocd-image-updater.argoproj.io/image-list: |
-      controller=ghcr.io/riotpiao-poimen/poimen-toolkit-controller
-    argocd-image-updater.argoproj.io/controller.update-strategy: digest
-    argocd-image-updater.argoproj.io/controller.allow-tags: regexp:^latest$
-    argocd-image-updater.argoproj.io/write-back-method: argocd
+    # Image promotion: Forgejo package webhook → api-gw.
+    # Tracks: poimen-toolkit-controller, pi-worker.
+    # Replaced ArgoCD Image Updater (removed 2026-09-10).
 spec:
   project: homelab
   source: