feat: add memory citation to agent message (#14821)

Client side to come

codex-rs/protocol/src/lib.rs

@@ -7,6 +7,7 @@ pub mod custom_prompts;
 pub mod dynamic_tools;
 pub mod items;
 pub mod mcp;
+pub mod memory_citation;
 pub mod message_history;
 pub mod models;
 pub mod num_format;