chore(core) rm Feature::RequestRule (#11866)

## Summary
This feature is now reasonably stable, let's remove it so we can
simplify our upcoming iterations here.

## Testing 
- [x] Existing tests pass

codex-rs/core/src/features.rs

@@ -511,8 +511,8 @@ pub const FEATURES: &[FeatureSpec] = &[
     FeatureSpec {
         id: Feature::RequestRule,
         key: "request_rule",
-        stage: Stage::Stable,
-        default_enabled: true,
+        stage: Stage::Removed,
+        default_enabled: false,
     },
     FeatureSpec {
         id: Feature::WindowsSandbox,