Let plugins choose pane placement (inline vs dock) and frame style

Plugins can't choose where a Pane sits or how its frame looks, so a pane can't keep one consistent design across layouts.

What happens now

• In fullscreen from 110 columns, every Pane docks on the right, floor to ceiling
• Below that, or on the main screen, the same Pane opens inline above the prompt
• placement is read-only on the Pane props, and the frame is drawn by the engine with no style option

Why it matters

I built a request board as a mod. It reads much better as a wide box above the prompt than as a narrow sidebar. Right now the only way to get that in fullscreen is to rebuild it as an AbovePrompt band. That means giving up Esc-to-close, ctrl+x tab and the engine's close mark.

What would help

• placement?: 'auto' | 'inline' | 'dock' on $.ui.open, with auto as today's behaviour
• frame?: 'round' | 'single' | 'none' so a plugin can soften the frame or draw its own

Setup

• Claude Code 2.1.289
• Windows 11, fullscreen layout (tui: fullscreen)
• Related: #91870