feat: split memories part 2 (#19860)

Keep extracting memories out of core and moving the write trigger in the
app-server
This is temporary and it should move at the client level as a follow-up
This makes core fully independant from `codex-memories-write`

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/thread-store/src/remote/helpers.rs

@@ -124,6 +124,7 @@ pub(super) fn proto_session_source(source: &SessionSource) -> proto::SessionSour
             sub_agent_other: Some(other.clone()),
             ..Default::default()
         },
+        SessionSource::Internal(_) => proto_source(proto::SessionSourceKind::Unknown),
         SessionSource::Unknown => proto_source(proto::SessionSourceKind::Unknown),
     }
 }