Add computer_use feature requirement key (#19071)

## Summary
- add the `computer_use` requirements-only feature key
- include it in generated config schema output
- cover the new key in feature metadata tests

## Testing
- `cargo test -p codex-features`
- `just write-config-schema`
- `just fmt`
- `just fix -p codex-features`

cc @xl-openai

---------

Co-authored-by: Dylan Hurd <dylan.hurd@openai.com>

codex-rs/features/src/tests.rs

@@ -153,6 +153,10 @@ fn browser_controls_are_stable_and_enabled_by_default() {
     assert_eq!(Feature::BrowserUse.stage(), Stage::Stable);
     assert_eq!(Feature::BrowserUse.default_enabled(), true);
     assert_eq!(feature_for_key("browser_use"), Some(Feature::BrowserUse));
+
+    assert_eq!(Feature::ComputerUse.stage(), Stage::Stable);
+    assert_eq!(Feature::ComputerUse.default_enabled(), true);
+    assert_eq!(feature_for_key("computer_use"), Some(Feature::ComputerUse));
 }
 
 #[test]