refactor(agent): unify sourced message delivery

docs/core-data-structures/session-reference.md

@@ -44,7 +44,7 @@ interface PreparedReferencedMessage {
   /** Readable message content after host mention tokens are removed. */
   content: ContentBlock[]
   /** Aggregated untrusted snapshot, absent when the message has no references. */
-  additionalContext?: AdditionalContext
+  additionalContext?: UserMessageData
 }
 ```