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
   ];
 }