straylight: add herdr

Pin herdr v0.9.0 from the upstream flake (nixpkgs-unstable only has
0.7.1) and install it on straylight to host coding-agent terminals.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

hw/straylight/default.nix

@@ -116,6 +116,12 @@ in
     programs.steam.enable = true;
     programs.steam.protontricks.enable = true;
 
+    # herdr: terminal workspace manager for coding agents (https://herdr.dev).
+    # Pinned to the upstream flake (v0.9.0); nixpkgs-unstable only has 0.7.1.
+    environment.systemPackages = [
+      inputs.herdr.packages.x86_64-linux.default
+    ];
+
     services.open-webui = {
       enable = true;
       host = "0.0.0.0";