Show `/side` chats in a split pane instead of replacing the main chat

What variant of Codex are you using?

Codex CLI (macOS)

What feature would you like to see?

When opening a temporary side chat with /side, show it alongside the main conversation in a split view instead of replacing the entire conversation area.

Currently, /side takes over the main chat view. The only obvious way to return is to cancel or close the side chat. That makes it harder to reference the main conversation while asking a focused side question, even though keeping the main task visible is the core reason to use a side chat.

A side-by-side layout could:

• keep the main conversation visible in one pane;
• open the ephemeral side chat in a second, optionally resizable pane;
• let the user close the side pane without navigating away from the main chat; and
• fall back to the current full-width presentation on narrow windows.

This could be the default behavior or a user-selectable presentation mode.

Additional information

I searched the open and closed issues for /side, side chats, split view, split pane, side-by-side, and overlay behavior. I found related requests about side-chat persistence/history, but no issue specifically requesting a split-pane UI.