fix: deprecate use_legacy_landlock feature flag (#17971)

## Summary
- mark `features.use_legacy_landlock` as a deprecated feature flag
- emit a startup deprecation notice when the flag is configured
- add feature- and core-level regression coverage for the notice


<img width="1288" height="93" alt="Screenshot 2026-04-15 at 11 14 00 PM"
src="https://github.com/user-attachments/assets/fffc628b-614c-4521-9374-64e50a269252"
/>

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/features/src/tests.rs

@@ -42,8 +42,8 @@ fn default_enabled_features_are_stable() {
 }
 
 #[test]
-fn use_legacy_landlock_is_stable_and_disabled_by_default() {
-    assert_eq!(Feature::UseLegacyLandlock.stage(), Stage::Stable);
+fn use_legacy_landlock_is_deprecated_and_disabled_by_default() {
+    assert_eq!(Feature::UseLegacyLandlock.stage(), Stage::Deprecated);
     assert_eq!(Feature::UseLegacyLandlock.default_enabled(), false);
 }
 
@@ -166,6 +166,28 @@ fn image_generation_is_stable_and_enabled_by_default() {
     assert_eq!(Feature::ImageGeneration.default_enabled(), true);
 }
 
+#[test]
+fn use_legacy_landlock_config_records_deprecation_notice() {
+    let mut entries = BTreeMap::new();
+    entries.insert("use_legacy_landlock".to_string(), true);
+
+    let mut features = Features::with_defaults();
+    features.apply_map(&entries);
+
+    let usages = features.legacy_feature_usages().collect::<Vec<_>>();
+    assert_eq!(usages.len(), 1);
+    assert_eq!(usages[0].alias, "features.use_legacy_landlock");
+    assert_eq!(usages[0].feature, Feature::UseLegacyLandlock);
+    assert_eq!(
+        usages[0].summary,
+        "`[features].use_legacy_landlock` is deprecated and will be removed soon."
+    );
+    assert_eq!(
+        usages[0].details.as_deref(),
+        Some("Remove this setting to stop opting into the legacy Linux sandbox behavior.")
+    );
+}
+
 #[test]
 fn image_detail_original_is_a_removed_feature_key() {
     assert_eq!(