Add collaboration modes test prompts (#9443)

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codex-rs/core/src/features.rs

@@ -100,6 +100,8 @@ pub enum Feature {
     Collab,
     /// Steer feature flag - when enabled, Enter submits immediately instead of queuing.
     Steer,
+    /// Enable collaboration modes (Plan, Pair Programming, Execute).
+    CollaborationModes,
 }
 
 impl Feature {
@@ -444,4 +446,10 @@ pub const FEATURES: &[FeatureSpec] = &[
         },
         default_enabled: false,
     },
+    FeatureSpec {
+        id: Feature::CollaborationModes,
+        key: "collaboration_modes",
+        stage: Stage::Experimental,
+        default_enabled: false,
+    },
 ];