[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/features/src/lib.rs

@@ -132,8 +132,6 @@ pub enum Feature {
     LocalThreadStoreCompression,
     /// Enable the Chronicle sidecar for passive screen-context memories.
     Chronicle,
-    /// Append additional AGENTS.md guidance to user instructions.
-    ChildAgentsMd,
     /// Compress request bodies (zstd) when sending streaming requests to codex-backend.
     EnableRequestCompression,
     /// Start the managed network proxy for sandboxed sessions.
@@ -910,12 +908,6 @@ pub const FEATURES: &[FeatureSpec] = &[
         stage: Stage::UnderDevelopment,
         default_enabled: false,
     },
-    FeatureSpec {
-        id: Feature::ChildAgentsMd,
-        key: "child_agents_md",
-        stage: Stage::UnderDevelopment,
-        default_enabled: false,
-    },
     FeatureSpec {
         id: Feature::ApplyPatchFreeform,
         key: "apply_patch_freeform",

codex-rs/features/src/tests.rs

@@ -707,12 +707,15 @@ fn materialize_resolved_enabled_writes_all_features_and_preserves_custom_config(
 #[test]
 fn unstable_warning_event_only_mentions_enabled_under_development_features() {
     let mut configured_features = Table::new();
-    configured_features.insert("child_agents_md".to_string(), TomlValue::Boolean(true));
+    configured_features.insert(
+        "apply_patch_streaming_events".to_string(),
+        TomlValue::Boolean(true),
+    );
     configured_features.insert("personality".to_string(), TomlValue::Boolean(true));
     configured_features.insert("unknown".to_string(), TomlValue::Boolean(true));
 
     let mut features = Features::with_defaults();
-    features.enable(Feature::ChildAgentsMd);
+    features.enable(Feature::ApplyPatchStreamingEvents);
 
     let warning = unstable_features_warning_event(
         Some(&configured_features),
@@ -725,7 +728,7 @@ fn unstable_warning_event_only_mentions_enabled_under_development_features() {
     let EventMsg::Warning(WarningEvent { message }) = warning.msg else {
         panic!("expected warning event");
     };
-    assert!(message.contains("child_agents_md"));
+    assert!(message.contains("apply_patch_streaming_events"));
     assert!(!message.contains("personality"));
     assert!(message.contains("/tmp/config.toml"));
 }