Feature Request: Support for custom color themes

## Feature Request: Custom Color Themes

### Problem
Currently, the `/theme` command only supports `auto`, `dark`, and `light` color modes. There is no way to customize the color palette used by the CLI (e.g., accent colors, background tints, text colors, etc.).

### Use Case
When running multiple Copilot CLI sessions with different configurations (e.g., a standard session vs. an admin session with a different model), it would be very helpful to visually distinguish them with distinct color themes. Right now, the only option is to change the terminal emulator's color scheme, which is a workaround rather than a first-class solution.

### Proposed Solution
Support custom themes via configuration, for example in `~/.copilot/config.json` or a dedicated `~/.copilot/themes/` directory. A theme could define colors for:

- Background/foreground
- Accent colors (prompt, highlights, borders)
- Status bar / footer colors
- Diff colors
- Agent output colors

Example config:
```json
{
"theme": "admin",
"themes": {
"admin": {
"accent": "#7C4DFF",
"background": "#1A0A2E",
"foreground": "#E0D0FF"
}
}
}
```

This would also allow users to share themes, which could build community engagement.

### Alternatives Considered
- Using Windows Terminal / iTerm2 color schemes as a workaround (current approach)
- Using `/theme dark` vs `/theme light` to differentiate sessions (limited)

### Additional Context
Running on Windows with multiple desktop shortcuts for different Copilot configurations (different models, admin vs. non-admin). Visual distinction at the Copilot level would be much cleaner than relying on terminal emulator theming.