begin migration to dendritic pattern

old-config/modules/zsh.nix

@@ -0,0 +1,9 @@
+{
+  config,
+  pkgs,
+  ...
+}: {
+  programs.zsh.enable = true;
+  users.defaultUserShell = pkgs.zsh;
+  users.users."grey".shell = pkgs.zsh;
+}