feat(agents): add design skill

modules/agents/skills.nix

@@ -12,10 +12,12 @@ let
   # The skills installed globally, as derivations from the skills flake.
   # wiki reads the personal Obsidian vault, and consume mines a source into it.
   # grill interviews the operator relentlessly to resolve a plan before building.
+  # design-skill drafts and audits Agent Skills for structural predictability.
   skills = with inputs.skills.packages.${pkgs.stdenv.hostPlatform.system}; [
     wiki
     consume
     grill
+    design-skill
   ];
 in
 {