fix(kugetsu): fix bash substitution error in cmd_start

The function call inside ${} syntax was invalid. Changed to use
command substitution $(...) instead.

skills/kugetsu/scripts/kugetsu

@@ -247,7 +247,7 @@ cmd_start() {
         exit 1
     fi
 
-    local session_file="${issue_ref_to_filename "$issue_ref"}.json"
+    local session_file="$(issue_ref_to_filename "$issue_ref").json"
 
     echo "Forking session for '$issue_ref'..."
     if [ "$DEBUG_MODE" = true ]; then