fix(shells): disable the nushell welcome banner without Home Manager

Assisted-by: DeepSeek Harness (deepseek-flash)

modules/nixos/nushell-login.nix

@@ -2,4 +2,14 @@
 {
   users.users.${myvars.username}.shell = pkgs.nushell;
   environment.shells = [ pkgs.nushell ];
+
+  # Hosts without Home Manager get no Nix-managed `config.nu`, so nushell
+  # generates one in `~/.config/nushell` with the welcome banner enabled; on a
+  # tmpfs `$HOME` that is a fresh banner after every boot. This vendor autoload
+  # is read after `config.nu`, so it turns the banner off without taking over
+  # the user's own config file.
+  programs.nushell = {
+    enable = true;
+    interactiveShellInit = "$env.config.show_banner = false";
+  };
 }