feat(ai-harness): add ponytail plugin

modules/nixos/services/hermes-agent.nix

@@ -3,6 +3,7 @@
     {
       config,
       lib,
+      pkgs,
       options,
       userName,
       ...
@@ -30,6 +31,15 @@
           };
           environmentFiles = cfg.environmentFiles;
           addToSystemPackages = true;
+
+          extraPlugins = [
+            (pkgs.fetchFromGitHub {
+              owner = "DietrichGebert";
+              repo = "ponytail";
+              rev = "v4.9.0";
+              hash = "sha256-8cYggVltBAlZ/Zj4pl1bOu7mQdZFXCmDGW4RSpvRA+w=";
+            })
+          ];
         };
 
         users.users.${userName}.extraGroups = [ "hermes" ];