opencode --help unreadable on light themes

`opencode --help` uses `#FFFFFF` as a foreground color, which makes it unreadable on light themes. It should use the terminal's configured foreground color instead of an absolute hex code.

<img width="598" alt="Image" src="https://github.com/user-attachments/assets/3ae3c8c7-0924-4a78-8731-9fa88a5ab7eb" />