chore: rename collab feature flag key to multi_agent (#11918)

Summary
- rename the collab feature key to multi_agent while keeping the Feature
enum unchanged
- add legacy alias support so both "multi_agent" and "collab" map to the
same feature
- cover the alias behavior with a new unit test

codex-rs/core/src/features.rs

@@ -552,7 +552,7 @@ pub const FEATURES: &[FeatureSpec] = &[
     },
     FeatureSpec {
         id: Feature::Collab,
-        key: "collab",
+        key: "multi_agent",
         stage: Stage::Experimental {
             name: "Sub-agents",
             menu_description: "Ask Codex to spawn multiple agents to parallelize the work and win in efficiency.",
@@ -738,4 +738,10 @@ mod tests {
         );
         assert_eq!(Feature::UseLinuxSandboxBwrap.default_enabled(), false);
     }
+
+    #[test]
+    fn collab_is_legacy_alias_for_multi_agent() {
+        assert_eq!(feature_for_key("multi_agent"), Some(Feature::Collab));
+        assert_eq!(feature_for_key("collab"), Some(Feature::Collab));
+    }
 }