Running under WSL, when copying text, BOM is always prepended to any copied text segment

### Describe the bug

Running under WSL, when copying text, BOM (0xFEFF) is always prepended to any copied text segment.

e.g.
```
Then that strongly points to a Copilot CLI copy bug.

If it happens:

- only when copying from Copilot CLI output
- regardless of where the selection starts
- and always as a leading zero-width character

then the most likely explanation is that the UI/renderer is placing a hidden U+FEFF/BOM at the start of
copied selections.

So yes — based on what you described, I’d treat this as a bug in Copilot CLI’s copy/export behavior, not
in WSL or Bash.
```

### Affected version

GitHub Copilot CLI 1.0.21.

### Steps to reproduce the behavior

1. Install ubuntu under WSL
2. Install homebrew
3. `brew install copilot-cli`
4. `copilot`
5. Highlight some text output from the copilot app
6. Right-click
7. Paste the output into an editor that shows zero-width characters
8. Text is prepended with 0XFEFF

### Expected behavior

Text is _not_ prepended with 0XFEFF

### Additional context

_No response_

首先按照列出的安装和复制步骤，在 Ubuntu 上的 WSL 中使用 GitHub Copilot CLI 1.0.21 复现该问题。比较粘贴的选区是否带有开头的 U+FEFF 字符；当复制的文本在能够显示零宽字符的编辑器中不再以 BOM 开头时，即视为完成。

由索引模型根据 Issue 内容生成。