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/codex.nix

@@ -0,0 +1,17 @@
+{
+  pkgs,
+  lib,
+  config,
+  ...
+}: {
+  options.codex.enable = lib.mkEnableOption "Codex";
+
+  config.programs.codex = lib.mkIf config.codex.enable {
+    enable = true;
+    package = pkgs.codex;
+    # skills = {
+    #   ponytail = /home/${username}/.agents/skills/ponytail;
+    #   caveman = /home/${username}/.agents/skills/caveman;
+    # };
+  };
+}