Add Cerebras PAUG provider and split models

Update default model provider to “Cerebras PAUG”. Reorganize the
Cerebras language‑model configuration into two groups – “Cerebras FREE”
and “Cerebras PAUG” – each with its own model list and adjusted token
limits.

modules/home/zed/user-settings/agent.nix

@@ -3,7 +3,7 @@ _: {
     always_allow_tool_actions = true;
     default_profile = "ask";
     default_model = {
-      provider = "Cerebras";
+      provider = "Cerebras PAUG";
       model = "gpt-oss-120b";
     };
   };