chore: collab experimental (#10381)

codex-rs/core/src/features.rs

@@ -520,7 +520,11 @@ pub const FEATURES: &[FeatureSpec] = &[
     FeatureSpec {
         id: Feature::Collab,
         key: "collab",
-        stage: Stage::UnderDevelopment,
+        stage: Stage::Experimental {
+            name: "Sub-agents",
+            menu_description: "Ask Codex to spawn multiple agents to parallelize the work and win in efficiency.",
+            announcement: "NEW: Sub-agents can now be spawned by Codex. Enable in /experimental and restart Codex!",
+        },
         default_enabled: false,
     },
     FeatureSpec {