Plugin panes: allow hiding or restyling the engine's close mark (✕)

Summary

A plugin that opens a pane with $.ui.open cannot hide, restyle or reposition the engine's close mark (✕) on the pane frame. A plugin that draws its own close control ends up with two close buttons.

What happens

• The ✕ is drawn on a row of the frame's own, outside the plugin's tree, so it cannot be aligned with the plugin's content.
• In my terminal the ✕ sits high above the content and is not aligned with it. Its hover highlight looks broken.
• PaneOpenArgs (id, title, focus, closeOnEscape, holdToasts, rows, columns) has no option to hide or move the mark.
• AbovePrompt is not an alternative: it draws its own [-] mark, and closeOnEscape applies only to panes.

Expected

An opt-out on $.ui.open, for example closeMark: false. A plugin that renders its own close control and handles Esc could then hide the engine's mark. A way to style or align the mark would also help.

Environment

• Claude Code 2.1.299, Windows 11, Windows Terminal
• Plugin: a third-party "extra-mods" menu pane