fix: cmd_destroy unbound variable $2

With set -u, using $2 without default causes error when called without arguments.

skills/kugetsu/scripts/kugetsu-session.sh

@@ -523,7 +523,7 @@ cmd_destroy() {
     local target="${1:-}"
     local force=false
     
-    if [ "$2" = "-y" ]; then
+    if [ "${2:-}" = "-y" ]; then
         force=true
     fi