This crate defines the "types" for the protocol used by Codex CLI, which includes both "internal types" for communication between `codex-core` and `codex-tui`, as well as "external types" used with `codex app-server`.
This crate should have minimal dependencies.
Ideally, we should avoid "material business logic" in this crate, as we can always introduce `Ext`-style traits to add functionality to types in other crates.