This PR introduces the ability for `core` to send `warnings` as it can send `errors. It also sends a warning on compaction. <img width="811" height="187" alt="image" src="https://github.com/user-attachments/assets/0947a42d-b720-420d-b7fd-115f8a65a46a" />
codex-protocol
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.