feat(config): add down-arrow binding for silent pj invocation on empty prompt

dot_config/fish/functions/pj.fish

@@ -12,7 +12,9 @@ function pj --description "fzf package.json script runner with auto-detected pac
     end
 
     if test -z "$pkg"
-        echo "pj: no package.json found in this tree" >&2
+        # PJ_QUIET: called from the down-arrow binding, where a missing
+        # package.json is the normal case and must not print anything.
+        test -n "$PJ_QUIET"; or echo "pj: no package.json found in this tree" >&2
         return 1
     end
     set -l root (path dirname $pkg)