[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/prompts/src/lib.rs

@@ -1,4 +1,3 @@
-mod agents;
 mod apply_patch;
 mod compact;
 mod goals;
@@ -7,7 +6,6 @@ mod realtime;
 mod review_exit;
 mod review_request;
 
-pub use agents::HIERARCHICAL_AGENTS_MESSAGE;
 pub use apply_patch::APPLY_PATCH_TOOL_INSTRUCTIONS;
 pub use compact::SUMMARIZATION_PROMPT;
 pub use compact::SUMMARY_PREFIX;