Allow users to customize prompt symbols and status line

## Feature Request

Allow users to customize the symbols and indicators shown in the Copilot CLI prompt, similar to how tools like Starship allow full prompt configuration.

## Context

The Copilot CLI prompt includes a git branch glyph (U+2387 ⎇) and dirty indicator. While these render, they may not look great depending on the user's font/terminal setup. There's currently no way to change them.

## Desired behavior

Allow users to configure prompt symbols via `~/.copilot/config.json` or a dedicated config, for example:

```json
{
"prompt": {
"branchIcon": "⎇",
"dirtyIndicator": "△",
"untrackedIndicator": "?",
"aheadIndicator": "⇡",
"behindIndicator": "⇣"
}
}
```

Or alternatively, allow users to disable the built-in prompt status line entirely and rely on their own shell prompt.

## Why

- Different fonts render glyphs differently — what looks good in one setup may look off in another
- Power users already have customized prompts (Starship, oh-my-posh, custom functions) and may prefer consistency
- Similar to Claude Code's `/statusline` configuration