Enable skill search shadow selection by default (#32780)

## What changed

Mark `skill_search` as stable and enable it by default so the app server runs
shadow skill selection and emits its experiment metrics.

GitOrigin-RevId: ea9da3b71bfb3be2093aac89ad3d3e388931901d

codex-rs/features/src/lib.rs

@@ -1195,8 +1195,8 @@ pub const FEATURES: &[FeatureSpec] = &[
     FeatureSpec {
         id: Feature::SkillSearch,
         key: "skill_search",
-        stage: Stage::UnderDevelopment,
-        default_enabled: false,
+        stage: Stage::Stable,
+        default_enabled: true,
     },
     FeatureSpec {
         id: Feature::SkillEnvVarDependencyPrompt,