Split features into codex-features crate (#15253)

- Split the feature system into a new `codex-features` crate.
- Cut `codex-core` and workspace consumers over to the new config and
warning APIs.

Co-authored-by: Ahmed Ibrahim <219906144+aibrahim-oai@users.noreply.github.com>
Co-authored-by: Codex <noreply@openai.com>

codex-rs/tui_app_server/src/tooltips.rs

@@ -1,4 +1,4 @@
-use codex_core::features::FEATURES;
+use codex_features::FEATURES;
 use codex_protocol::account::PlanType;
 use lazy_static::lazy_static;
 use rand::Rng;