[FEATURE] Multi-line editing in prompt area

## Feature Description

When entering a longish prompt, I would like to be able to split the instructions visually over multiple lines.

## Use Case

I believe this will make it easier to work with longer prompts. Example:

Please introduce feature foo. Do this by:
1. meeting requirement bar
2. meeting requirement baz

Is for me easier to read than:

Please introduce feature foo. Do this by: 1. meeting requirement bar 2. meeting requirement baz.

## Proposed Solution

I'm not sure how the prompt area is implemented under the hood, but I could image this either being a buffer that allows multiple lines to be entered (which would also allow more complex editing to be done slightly easier), or I could image that a keybind is introduced (e.g. shift enter) that causes a line break and then the possibility to continue to edit on the next line.

## Alternatives Considered

When copying pasting into the windows, multi line input is supported / can be pasted into the prompt area in the claude code terminal window in neovim, so a potential alternative is to construct prompts outside of the claude code buffer and then past them into the prompt area. This is clunky though, and for longer prompts can lead to the prompt showing that n lines have been pasted, but not showing all these lines (which is useful by itself, but makes this a less desirable alternative solution for me)

Issue にはファイル、テスト、エントリーポイントが記載されていないため、まず prompt-area の実装箇所を特定し、入力、貼り付けた複数行テキスト、キーバインディングが現在どのように動作するかを確認します。提案されているバッファ方式と Shift-Enter 方式を比較し、そのうえで機能を完成させるための編集および送信の挙動を定義します。

索引モデルが issue の本文から書いたものです。