feat: Image Updater git write-back for multi-source poimen Application

- write-back-method: git (commits image updates back to repos)
- git-branch: main
- Mounts ArgoCD SSH credentials for git pushes
- Image Updater commits new SHAs → repos → ArgoCD syncs

k8s/argocd/apps/70-poimen.yaml

@@ -16,7 +16,8 @@ metadata:
     argocd-image-updater.argoproj.io/workflows.allow-tags: regexp:^[0-9a-f]{7}$
     argocd-image-updater.argoproj.io/frontend.update-strategy: newest-build
     argocd-image-updater.argoproj.io/frontend.allow-tags: regexp:^[0-9a-f]{7}$
-    argocd-image-updater.argoproj.io/write-back-method: argocd
+    argocd-image-updater.argoproj.io/write-back-method: git
+    argocd-image-updater.argoproj.io/git-branch: main
 spec:
   project: homelab
   sources: