feat(pm/systems): move zsh to it's own file

packetmix/systems/freshlybakedcake/shell.nix

···
+# SPDX-FileCopyrightText: 2025 FreshlyBakedCake
+#
+# SPDX-License-Identifier: MIT
+
+{
+  programs.zsh.enable = true;
+}

packetmix/systems/freshlybakedcake/users.nix

···
       "[e-mail] AAAAGnNrLXNzaC1lZDI1NTE5QG9wZW5zc2guY29tAAAAIHgyfY0KnxIsw8h9/cmbZ8InRFMqOmWn94teVsAHMvNkAAAABHNzaDo= coded@OcicatNanoResident"
       "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIKpBNIHk/kRhQL7Nl3Fd+UBVRoS2bTpbeerA//vwL2D4 coded@passwordKey"
     ];
+
+    shell = pkgs.zsh;
   };
 
-  programs.zsh.enable = true;
   users.users.minion = {
     isNormalUser = true;
     description = "Skyler Grey";