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/default.nix

@@ -19,6 +19,7 @@ _: {
     ./btop.nix
     ./cargo.nix
     ./cava.nix
+    ./codex.nix
     ./easyeffects.nix
     ./eza.nix
     ./fzf.nix
@@ -37,6 +38,7 @@ _: {
     ./opencode.nix
     ./prismlauncher.nix
     ./qt.nix
+    ./skills.nix
     ./ssh.nix
     ./starship.nix
     ./stylix.nix