Update default model in LangGraphAgent to use OpenAI GPT-OSS 20B

agent.py

@@ -74,7 +74,7 @@ class LangGraphAgent:
         # Choose an LLM if not provided
         if model is None:
             model = ChatOpenRouter(
-                model="mistralai/mistral-small-3.2-24b-instruct:free")
+                model="openai/gpt-oss-20b:free")
             if model is None and ChatOpenAI is not None:
                 model = ChatOpenAI(
                     api_key=os.getenv("OPENROUTER_API_KEY"),