document filter_chain support on model listeners

docs/source/resources/includes/plano_config_full_reference.yaml

@@ -66,6 +66,9 @@ listeners:
     name: model_1
     address: 0.0.0.0
     port: 12000
+    # Optional: attach a filter chain for input guardrails on direct LLM requests
+    # filter_chain:
+    #   - input_guards
 
   # Prompt listener for function calling (for prompt_targets)
   - type: prompt