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/home/zed/default.nix

@@ -19,6 +19,5 @@
     ./user-settings
     ./extensions.nix
     ./extra-packages.nix
-    ./skills.nix
   ];
 }

modules/home/zed/skills.nix

@@ -1,20 +0,0 @@
-{
-  pkgs,
-  # lib,
-  ...
-}: let
-  skillPath = ".agents/skills";
-  urlDomain = "https://raw.githubusercontent.com";
-  refs = "refs/heads/main/skills";
-in {
-  home.file = {
-    "${skillPath}/ponytail/SKILL.md".source = pkgs.fetchurl {
-      url = "${urlDomain}/DietrichGebert/ponytail/${refs}/ponytail/SKILL.md";
-      sha256 = "sha256-IiTh4N3bHhJtER4HYPEFEo1g9Ro703TiPA5Tigv7k3s=";
-    };
-    "${skillPath}/caveman/SKILL.md".source = pkgs.fetchurl {
-      url = "${urlDomain}/JuliusBrussee/caveman/${refs}/caveman/SKILL.md";
-      sha256 = "sha256-447Gcey+5HziNBkL4SYV2vYKxmfXdbc0DUnQf09jx7w=";
-    };
-  };
-}