Feature request: Customizable prompt markers (❯ and ●)

### Describe the feature or problem you'd like to solve

When scrolling back through a long session, it's difficult to visually distinguish user prompts (`❯`) from Copilot responses (`●`). Both are small single-character markers that blend together, especially in sessions with lots of tool output.

### Proposed solution

Allow customization of the user input prompt marker and the assistant response marker via `config.json` or CLI flags. For example:

```json
{
"prompt_marker": "🟢 ❯",
"response_marker": "●"
}
```

Or even just supporting a config option to add a colored prefix, username, or emoji before the `❯` would help.

### Example prompts or workflows

In a long session with multiple tool calls, the terminal output can be hundreds of lines. Scrolling back to find where a specific prompt was entered requires visually scanning for the small `❯` character among tool output, status messages, and response markers. A more distinctive or customizable marker would make navigation much faster.

### Additional context

Ghostty and other modern terminals support rich Unicode rendering, so emoji or multi-character markers would render fine.