[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/src/agents_md.rs

@@ -26,8 +26,6 @@ use codex_config::merge_toml_values;
 use codex_config::project_root_markers_from_config;
 use codex_exec_server::ExecutorFileSystem;
 use codex_extension_api::UserInstructions;
-use codex_features::Feature;
-use codex_prompts::HIERARCHICAL_AGENTS_MESSAGE;
 use codex_utils_absolute_path::AbsolutePathBuf;
 use codex_utils_path_uri::PathUri;
 use std::io;
@@ -72,13 +70,6 @@ pub(crate) async fn load_project_instructions(
         }
     }
 
-    if config.features.enabled(Feature::ChildAgentsMd) {
-        loaded.entries.push(InstructionEntry {
-            contents: HIERARCHICAL_AGENTS_MESSAGE.to_string(),
-            provenance: InstructionProvenance::Internal,
-        });
-    }
-
     (!loaded.is_empty()).then_some(loaded)
 }