feat: sub-agent injection (#12152)

This PR adds parent-thread sub-agent completion notifications and change
the prompt of the model to prevent if from being confused

codex-rs/core/tests/suite/mod.rs

@@ -114,6 +114,7 @@ mod skills;
 mod sqlite_state;
 mod stream_error_allows_next_turn;
 mod stream_no_completed;
+mod subagent_notifications;
 mod text_encoding_fix;
 mod tool_harness;
 mod tool_parallelism;