Thinking/reasoning text is unreadable on dark backgrounds (hardcoded dim color)

### Describe the bug

The "Thinking…" reasoning text displayed during model inference is nearly invisible on dark terminal backgrounds. The text appears to use a hardcoded dark gray foreground that has insufficient contrast against dark backgrounds.

This started with a recent update (noticed on 1.0.64).

### Environment

- **OS:** Windows 11 + WSL2
- **Terminal:** Windows Terminal (dark color scheme)
- **Multiplexer:** tmux
- **Copilot CLI version:** 1.0.64
- **Model:** Claude Opus 4.6 (extended thinking enabled)

### Steps to reproduce

1. Use a dark terminal color scheme (default Windows Terminal dark, or similar)
2. Run `copilot` in tmux
3. Ask a question that triggers extended thinking/reasoning
4. Observe the "Thinking…" block text is nearly invisible

### Expected behavior

Thinking/reasoning text should have sufficient contrast on both dark and light backgrounds. Ideally it would:
- Use a semantic ANSI color (e.g., `brightBlack` / color 8) rather than a hardcoded RGB
- Or respect the terminal's dim attribute rendering
- Or be configurable via a theme/config setting

### Workaround

Bumping `brightBlack` in the Windows Terminal color scheme to `#808080` or lighter partially helps, but this affects all dim text globally.

### Related issues

- #3750 — Hardcoded colors break light themes (same root cause, opposite direction)
- #2123 — Feature request for per-element theming (would solve this)
- #1504 — Custom theme support request

Start by reproducing the "Thinking…" block in the Copilot CLI under dark and light terminal themes, including Windows Terminal with tmux. Trace where the reasoning text color is selected and compare it with the related issues #3750, #2123, and #1504. Done means the text has sufficient contrast on both dark and light backgrounds without requiring a global brightBlack change.

索引モデルが issue の本文から書いたものです。