Add Codex module and skills, update Helium

Enable Codex on Garand-Desktop and Garand-Laptop.
Keep it disabled on the default host.

Add a new `codex.nix` module and import it in the home
configuration. Introduce a shared `skills.nix` module that
downloads Ponytail and Caveman skill files for both
`.agents/skills` and `.codex/skills`. Update Helium to
0.19.3.1 with the correct hash. Remove the obsolete Zed
skills module.

modules/core/packages/appimages/helium.nix

@@ -4,8 +4,8 @@
   fetchurl,
 }: let
   pname = "helium";
-  version = "0.19.2.1";
-  hash = "sha256-oEVQo8fHC9rTrNOkQw7ajSr8C/cXOpxYpAP+q5UXCH8=";
+  version = "0.19.3.1";
+  hash = "sha256-Rg5tKlyG0DkhYkItGyF6NmMb2DD40h5WUYyWAgl5eQY=";
 
   src = fetchurl {
     url = "https://github.com/imputnet/helium-linux/releases/download/${version}/helium-${version}-x86_64.AppImage";
@@ -36,4 +36,3 @@ in
       maintainers = with lib.maintainers; [garand_plg];
     };
   }
-# https://github.com/imputnet/helium-linux/releases/download/0.11.7.1/helium-0.11.7.1-x86_64.AppImage