[codex] Remove child AGENTS.md prompt experiment (#28993)

## Why

`child_agents_md` is a disabled, under-development experiment that adds
a second model-visible explanation of hierarchical `AGENTS.md` behavior.
Keeping it leaves unused prompt, configuration, documentation, and test
surface.

## What changed

- remove the `ChildAgentsMd` feature and `child_agents_md` config schema
entry
- remove the hierarchical prompt asset, export, and instruction
injection
- remove feature-specific tests and documentation
- keep the generic unstable-feature warning coverage using
`apply_patch_streaming_events`

Normal project `AGENTS.md` discovery and composition are unchanged.

## Testing

- `just test -p codex-features`
- `just test -p codex-prompts`
- `just test -p codex-core agents_md`
- `just test -p codex-core unstable_features_warning`

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

@@ -1,97 +0,0 @@
-use codex_features::Feature;
-use codex_utils_path_uri::PathUri;
-use core_test_support::responses::ev_completed;
-use core_test_support::responses::ev_response_created;
-use core_test_support::responses::mount_sse_once;
-use core_test_support::responses::sse;
-use core_test_support::responses::start_mock_server;
-use core_test_support::test_codex::test_codex;
-
-const HIERARCHICAL_AGENTS_SNIPPET: &str =
-    "Files called AGENTS.md commonly appear in many places inside a container";
-
-#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
-async fn hierarchical_agents_appends_to_project_doc_in_user_instructions() {
-    let server = start_mock_server().await;
-    let resp_mock = mount_sse_once(
-        &server,
-        sse(vec![ev_response_created("resp1"), ev_completed("resp1")]),
-    )
-    .await;
-
-    let mut builder = test_codex()
-        .with_config(|config| {
-            config
-                .features
-                .enable(Feature::ChildAgentsMd)
-                .expect("test config should allow feature update");
-        })
-        .with_workspace_setup(|cwd, fs| async move {
-            let agents_md = cwd.join("AGENTS.md");
-            let agents_md_uri = PathUri::from_path(&agents_md)?;
-            fs.write_file(&agents_md_uri, b"be nice".to_vec(), /*sandbox*/ None)
-                .await?;
-            Ok::<(), anyhow::Error>(())
-        });
-    let test = builder
-        .build_with_remote_env(&server)
-        .await
-        .expect("build test codex");
-
-    test.submit_turn("hello").await.expect("submit turn");
-
-    let request = resp_mock.single_request();
-    let user_messages = request.message_input_texts("user");
-    let instructions = user_messages
-        .iter()
-        .find(|text| text.starts_with("# AGENTS.md instructions"))
-        .expect("instructions message");
-    assert!(
-        instructions.contains("be nice"),
-        "expected AGENTS.md text included: {instructions}"
-    );
-    let snippet_pos = instructions
-        .find(HIERARCHICAL_AGENTS_SNIPPET)
-        .expect("expected hierarchical agents snippet");
-    let base_pos = instructions
-        .find("be nice")
-        .expect("expected AGENTS.md text");
-    assert!(
-        snippet_pos > base_pos,
-        "expected hierarchical agents message appended after base instructions: {instructions}"
-    );
-}
-
-#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
-async fn hierarchical_agents_emits_when_no_project_doc() {
-    let server = start_mock_server().await;
-    let resp_mock = mount_sse_once(
-        &server,
-        sse(vec![ev_response_created("resp1"), ev_completed("resp1")]),
-    )
-    .await;
-
-    let mut builder = test_codex().with_config(|config| {
-        config
-            .features
-            .enable(Feature::ChildAgentsMd)
-            .expect("test config should allow feature update");
-    });
-    let test = builder
-        .build_with_remote_env(&server)
-        .await
-        .expect("build test codex");
-
-    test.submit_turn("hello").await.expect("submit turn");
-
-    let request = resp_mock.single_request();
-    let user_messages = request.message_input_texts("user");
-    let instructions = user_messages
-        .iter()
-        .find(|text| text.starts_with("# AGENTS.md instructions"))
-        .expect("instructions message");
-    assert!(
-        instructions.contains(HIERARCHICAL_AGENTS_SNIPPET),
-        "expected hierarchical agents message appended: {instructions}"
-    );
-}

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

@@ -57,7 +57,6 @@ mod extension_sandbox;
 mod fork_thread;
 #[cfg(not(target_os = "windows"))]
 mod guardian_review;
-mod hierarchical_agents;
 #[cfg(not(target_os = "windows"))]
 mod hooks;
 #[cfg(not(target_os = "windows"))]

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

@@ -21,14 +21,14 @@ async fn emits_warning_when_unstable_features_enabled_via_config() {
     let mut config = load_default_config_for_test(&home).await;
     config
         .features
-        .enable(Feature::ChildAgentsMd)
+        .enable(Feature::ApplyPatchStreamingEvents)
         .expect("test config should allow feature update");
     let user_config_path =
         AbsolutePathBuf::from_absolute_path(config.codex_home.join(CONFIG_TOML_FILE))
             .expect("absolute user config path");
     config.config_layer_stack = config.config_layer_stack.with_user_config(
         &user_config_path,
-        toml! { features = { child_agents_md = true } }.into(),
+        toml! { features = { apply_patch_streaming_events = true } }.into(),
     );
 
     let thread_manager = codex_core::test_support::thread_manager_with_models_provider(
@@ -56,7 +56,7 @@ async fn emits_warning_when_unstable_features_enabled_via_config() {
     let EventMsg::Warning(WarningEvent { message }) = warning else {
         panic!("expected warning event");
     };
-    assert!(message.contains("child_agents_md"));
+    assert!(message.contains("apply_patch_streaming_events"));
     assert!(message.contains("Under-development features enabled"));
     assert!(message.contains("suppress_unstable_features_warning = true"));
 }
@@ -67,7 +67,7 @@ async fn suppresses_warning_when_configured() {
     let mut config = load_default_config_for_test(&home).await;
     config
         .features
-        .enable(Feature::ChildAgentsMd)
+        .enable(Feature::ApplyPatchStreamingEvents)
         .expect("test config should allow feature update");
     config.suppress_unstable_features_warning = true;
     let user_config_path =
@@ -75,7 +75,7 @@ async fn suppresses_warning_when_configured() {
             .expect("absolute user config path");
     config.config_layer_stack = config.config_layer_stack.with_user_config(
         &user_config_path,
-        toml! { features = { child_agents_md = true } }.into(),
+        toml! { features = { apply_patch_streaming_events = true } }.into(),
     );
 
     let thread_manager = codex_core::test_support::thread_manager_with_models_provider(