fix: stabilize kubeadm bootstrap and reduce Proxmox plan latency

Move kubeadm reset ahead of kube-vip manifest generation, use super-admin.conf during bootstrap for kube-vip, and restore admin.conf after init. Also switch nixos-rebuild to --sudo and make QEMU guest agent optional so Terraform plan can skip slow guest-agent refreshes when it is not installed.

nixos/kubeadm/scripts/rebuild-and-bootstrap.sh

@@ -164,7 +164,7 @@ rebuild_node() {
   timeout "$REBUILD_TIMEOUT" nixos-rebuild switch \
     --flake "$FLAKE_DIR#$node_name" \
     --target-host "$ACTIVE_SSH_USER@$node_ip" \
-    --use-remote-sudo
+    --sudo
 }
 
 rebuild_node_with_retry() {