doc: from_env in default_config.yaml

Author: Winston-503

configs/default_config.yaml

@@ -39,6 +39,9 @@ llm:
   # API configuration
   api_base: "https://generativelanguage.googleapis.com/v1beta/openai/"  # Base URL for API (change for non-OpenAI models)
   api_key: null                       # API key (defaults to OPENAI_API_KEY env variable)
+  # Or use from_env to specify which environment variable to read from:
+  # api_key:
+  #   from_env: GEMINI_API_KEY        # Reads API key from $GEMINI_API_KEY
 
   # Generation parameters
   temperature: 0.7                    # Temperature for generation (higher = more creative)