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.

hosts/default/home-modules.nix

@@ -8,6 +8,7 @@ _: {
     remote-server.enable = false; # Remote Server: enable remote editing capabilities
   };
   opencode.enable = false; # OpenCode: terminal coding agent
+  codex.enable = false; # Codex: Lightweight coding agent that runs in your terminal
   ollama.enable = false; # Ollama: Local Llms
 
   /*