feat(nixos): migrate system configuration

configs/nixos/modules/agents/skills.nix

@@ -0,0 +1,18 @@
+{
+  config,
+  inputs,
+  pkgs,
+  ...
+}:
+# Global agent skills, placed under the skills directory so they are active in
+# every project.
+let
+  user = config.user.name;
+
+  # The skills installed globally, as derivations from the skills flake.
+  skills = with inputs.skills.packages.${pkgs.stdenv.hostPlatform.system}; [ ];
+in
+{
+  home-manager.sharedModules = [ inputs.skills.homeModules.default ];
+  home-manager.users.${user}.programs.agents.skills = skills;
+}