theme: "auto" fails to detect dark background in iTerm2 — no color distinction

### Describe the bug

`theme: "auto"` produces no color distinction in iTerm2 — all text appears in similar muted tones.
Setting `theme: "dark"` explicitly in `~/.copilot/config.json` restores correct colors immediately.

This is a regression from ~2 versions ago. Same iTerm2 config, same shell (zsh + oh-my-zsh),
colors worked fine before.

### Steps to reproduce
1. Use iTerm2 with any dark color scheme
2. Have `"theme": "auto"` in config (the default)
3. Launch `copilot` — output has no color distinction

### Workaround
Set `"theme": "dark"` explicitly in `~/.copilot/config.json`.

### Environment
- Copilot CLI: 1.0.10
- macOS (Darwin), Apple Silicon
- iTerm2 3.6.9
- TERM=xterm-256color (profile), xterm-color (in session)
- COLORTERM=truecolor
- COLORFGBG=15;0 (correctly indicates dark bg)
- Shell: zsh + oh-my-zsh
- Terminal.app with same shell config works fine with `auto`