feat: add support for AGENTS.md in Rust CLI (#885)

The TypeScript CLI already has support for including the contents of
`AGENTS.md` in the instructions sent with the first turn of a
conversation. This PR brings this functionality to the Rust CLI.

To be considered, `AGENTS.md` must be in the `cwd` of the session, or in
one of the parent folders up to a Git/filesystem root (whichever is
encountered first).

By default, a maximum of 32 KiB of `AGENTS.md` will be included, though
this is configurable using the new-in-this-PR `project_doc_max_bytes`
option in `config.toml`.

codex-rs/core/src/lib.rs

@@ -28,6 +28,7 @@ mod model_provider_info;
 pub use model_provider_info::ModelProviderInfo;
 pub use model_provider_info::WireApi;
 mod models;
+mod project_doc;
 pub mod protocol;
 mod rollout;
 mod safety;