Colors are completely off in tmux

### Describe the bug

When having a light theme, Copilot CLI light theme's colors are completely wrong when running in tmux. Outside tmux, everything's fine.

tmux vs regular shell:

The problem isn't just the aesthetics, some text is genuinely unreadable (like bright yellow on light background in the screenshot above).

### Affected version

GitHub Copilot CLI 1.0.61

### Steps to reproduce the behavior

- Set a light terminal theme
- Set the theme to "light" or "auto" in Copilot CLI
- Spawn the command menu with "/"

### Expected behavior

Exactly the same color in and outside tmux.

### Additional context

Env:
- WSL2, NixOS
- Terminal emulator: Windows Terminal
- Fish shell

In tmux:
```shell
❯ echo TERM=$TERM; tput colors
TERM=tmux-direct
16777216

❯ echo $COLORTERM
truecolor
```

Relevant tmux config:
```
# Enable true color support
set-option -as terminal-features ',*:RGB'
set -ga update-environment "COLORTERM"
set -ga update-environment "COLORFGBG"
set-environment -g COLORTERM "truecolor"
```

Issue #4212 seems related but I'm not sure if it's exactly the same. There is also issue #135 but it hasn't received any attention in months.

Reproduce the light-theme color mismatch using Copilot CLI 1.0.61 inside and outside tmux, following the listed WSL2, NixOS, Windows Terminal, and Fish setup. Start by comparing the tmux environment values and configuration with issue #4212 and issue #135. Done means the light or auto theme displays the same readable colors when the command menu is opened with "/" in and outside tmux.

Written by the indexing model from the issue text.