Prompt box and highlighted menu items render invisible (dark-on-dark) inside tmux

### Describe the bug

Copilot CLI's prompt input box and highlighted/selected menu list items are unreadable (dark text on dark/matching background) when run inside tmux. The same session works correctly in a plain iTerm2 tab with no tmux.

During troubleshooting, ruled out the following:

• Stale  COLORFGBG  env var — unset it, no change.
• iTerm2 automatic light/dark profile switching — disabled it, no change.
• tmux truecolor passthrough — confirmed via  tmux info  that  Tc / RGB  report  true  after adding  set -ga terminal-overrides ",xterm-256color:Tc" , and  tmux capture-pane -e  shows correct 24-bit RGB SGR codes are being preserved (not quantized), so this isn't a color-depth/passthrough issue.
•  tmux allow-passthrough on  is enabled; OSC 11 background-color queries do get a reply (confirmed manually with  printf '\033]11;?\033\\' ), though the reply appears to arrive after Copilot's internal detection timeout in some tests.
• Restarting tmux server fully, restarting iTerm2 fully — no change.
• Switching Copilot's  theme  setting between  default ,  dim ,  github  — same problem persists in all of them, only inside tmux.

### Affected version

GitHub Copilot CLI 1.0.73. Run 'copilot update' to check for updates.

### Steps to reproduce the behavior

1. Launch iTerm2, start a tmux session, run  copilot .
2. Type in the prompt input box, or open a menu with selectable items (e.g.  /theme ).
3. Text in the prompt box / highlighted menu rows is invisible — same foreground/background color.

### Expected behavior

The colors should be legible regardless of being run inside tmux or not

### Additional context

_No response_

报告中未指出源文件或测试。请在 iTerm2 和 tmux 中使用 GitHub Copilot CLI 1.0.73 重现，与普通的 iTerm2 标签页进行比较，并检查报告的 truecolor 和 OSC 11 行为。当提示文本和高亮菜单项在所列主题下的 tmux 中都保持清晰可读时，即视为完成。

由索引模型根据 Issue 内容生成。