feat(opencode): add fast model to Halo provider

Adds a "fast" model option alongside the existing "coder" model in the
Halo provider configuration for opencode.

modules/home/cli-apps/opencode/default.nix

@@ -24,6 +24,7 @@ let
       name = "Halo";
       options.baseURL = "http://halo:8000/v1";
       models.coder.name = "coder";
+      models.fast.name = "fast";
     };
   };