feat(agent-skills): add the grill skill

Install the grill skill globally from the skills flake, alongside wiki and
consume. It interviews the operator relentlessly about a plan or design,
resolving every branch to an explicit decision before any implementation.

modules/agents/skills.nix

@@ -11,9 +11,11 @@ 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.
   skills = with inputs.skills.packages.${pkgs.stdenv.hostPlatform.system}; [
     wiki
     consume
+    grill
   ];
 in
 {