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/core/tests/suite/undo.rs

@@ -9,7 +9,7 @@ use anyhow::Context;
 use anyhow::Result;
 use anyhow::bail;
 use codex_core::CodexThread;
-use codex_core::features::Feature;
+use codex_features::Feature;
 use codex_protocol::protocol::EventMsg;
 use codex_protocol::protocol::Op;
 use codex_protocol::protocol::UndoCompletedEvent;