Enable ghost_commit feature by default (#6041)

## Summary
- enable the ghost_commit feature flag by default

## Testing
- just fmt

------
https://chatgpt.com/codex/tasks/task_i_6904ce2d0370832dbb3c2c09a90fb188

codex-rs/core/src/features.rs

@@ -284,7 +284,7 @@ pub const FEATURES: &[FeatureSpec] = &[
         id: Feature::GhostCommit,
         key: "ghost_commit",
         stage: Stage::Experimental,
-        default_enabled: false,
+        default_enabled: true,
     },
     FeatureSpec {
         id: Feature::WindowsSandbox,