add: clarifying comment on credit usage

enterprise/app/helpers/captain/chat_response_helper.rb

@@ -35,6 +35,9 @@ module Captain::ChatResponseHelper
 
   def credit_used_for_response?(parsed_response)
     response = parsed_response['response']
+
+    # The classifier can still decide to hand off after this trace is written.
+    # Actual response usage is charged later in ResponseBuilderJob, so billing stays correct.
     response.present? && response != 'conversation_handoff' && parsed_response['action'] != 'handoff'
   end