feat(core) RequestRule (#9489)

## Summary
Instead of trying to derive the prefix_rule for a command mechanically,
let's let the model decide for us.

## Testing
- [x] tested locally

codex-rs/core/src/features.rs

@@ -89,6 +89,8 @@ pub enum Feature {
     WebSearchCached,
     /// Gate the execpolicy enforcement for shell/unified exec.
     ExecPolicy,
+    /// Allow the model to request approval and propose exec rules.
+    RequestRule,
     /// Enable Windows sandbox (restricted token) on Windows.
     WindowsSandbox,
     /// Use the elevated Windows sandbox pipeline (setup + runner).
@@ -393,6 +395,12 @@ pub const FEATURES: &[FeatureSpec] = &[
         stage: Stage::UnderDevelopment,
         default_enabled: true,
     },
+    FeatureSpec {
+        id: Feature::RequestRule,
+        key: "request_rule",
+        stage: Stage::UnderDevelopment,
+        default_enabled: false,
+    },
     FeatureSpec {
         id: Feature::WindowsSandbox,
         key: "experimental_windows_sandbox",