tmux: use fish directly on nixos, zsh on macos

home/modules/tmux.nix

@@ -5,6 +5,7 @@ let
 
   cfg = config.snowhawk.tmux;
   sessionizer = cfg.sessionizer;
+  macos = config.snowhawk.macos.enable;
 in
 {
   options.snowhawk.tmux = {
@@ -14,7 +15,7 @@ in
   config = lib.mkIf cfg.enable {
     programs.tmux = {
       enable = true;
-      shell = getExe pkgs.zsh;
+      shell = getExe (if macos then pkgs.zsh else pkgs.fish);
       mouse = true;
       baseIndex = 1;
       terminal = "screen-256color";