:q closes the active editor tab and focuses the next; the last :q
collapses the split for free via the existing editor.active-changed{id:null}
→ arrangement.closeEditor() path, so :q never dispatches command:editor.close.
The one gap is registry close() re-focusing first-not-next; recommend the
UI-side next-tab activate-then-close.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>