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/src/windows_sandbox_tests.rs

@@ -1,7 +1,7 @@
 use super::*;
 use crate::config::types::WindowsToml;
-use crate::features::Features;
-use crate::features::FeaturesToml;
+use codex_features::Features;
+use codex_features::FeaturesToml;
 use pretty_assertions::assert_eq;
 use std::collections::BTreeMap;