fix: auto-detect kube-vip interface and tighten SSH fallback

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

@@ -86,6 +86,7 @@ remote() {
   local quoted_cmd
   local candidate
   local candidates=()
+  local rc=0
 
   candidates+=("$ACTIVE_SSH_USER")
   for candidate in $SSH_USER_CANDIDATES; do
@@ -100,6 +101,11 @@ remote() {
       ACTIVE_SSH_USER="$candidate"
       return 0
     fi
+
+    rc=$?
+    if [ "$rc" -ne 255 ]; then
+      return "$rc"
+    fi
   done
 
   echo "Remote command failed for all SSH users on $host_ip"