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/skills.nix

@@ -0,0 +1,27 @@
+{
+  pkgs,
+  # lib,
+  ...
+}: let
+  skillPath = ".agents/skills";
+  skillCodexPath = ".codex/skills";
+  urlDomain = "https://raw.githubusercontent.com";
+  refs = "refs/heads/main/skills";
+
+  ponytailSkill = pkgs.fetchurl {
+    url = "${urlDomain}/DietrichGebert/ponytail/${refs}/ponytail/SKILL.md";
+    sha256 = "sha256-IiTh4N3bHhJtER4HYPEFEo1g9Ro703TiPA5Tigv7k3s=";
+  };
+
+  cavemanSkill = pkgs.fetchurl {
+    url = "${urlDomain}/JuliusBrussee/caveman/${refs}/caveman/SKILL.md";
+    sha256 = "sha256-447Gcey+5HziNBkL4SYV2vYKxmfXdbc0DUnQf09jx7w=";
+  };
+in {
+  home.file = {
+    "${skillPath}/ponytail/SKILL.md".source = ponytailSkill;
+    "${skillPath}/caveman/SKILL.md".source = cavemanSkill;
+    "${skillCodexPath}/ponytail/SKILL.md".source = ponytailSkill;
+    "${skillCodexPath}/caveman/SKILL.md".source = cavemanSkill;
+  };
+}