enable plan mode (#10313)

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codex-rs/core/src/features.rs

@@ -558,8 +558,8 @@ pub const FEATURES: &[FeatureSpec] = &[
     FeatureSpec {
         id: Feature::CollaborationModes,
         key: "collaboration_modes",
-        stage: Stage::UnderDevelopment,
-        default_enabled: false,
+        stage: Stage::Stable,
+        default_enabled: true,
     },
     FeatureSpec {
         id: Feature::Personality,