read all AGENTS.md up to git root (#2532)

This updates our logic for AGENTS.md to match documented behavior, which
is to read all AGENTS.md files from cwd up to git root.

codex-rs/core/src/lib.rs

@@ -43,7 +43,7 @@ mod models;
 mod openai_model_info;
 mod openai_tools;
 pub mod plan_tool;
-mod project_doc;
+pub mod project_doc;
 mod rollout;
 pub(crate) mod safety;
 pub mod seatbelt;