Toggle should preserve buffers when keep_buffers_on_toggle enabled

[jensenojs]

Current behavior:

• :Opencode toggle closes windows and deletes input/output buffers
• Reopening recreates the UI, re-renders output, and loses input/scroll position

Expected behavior:

• When ui.keep_buffers_on_toggle = true, toggle should hide the UI without deleting buffers
• Reopening should restore the existing buffers