Allow users to customize prompt symbols and status line

[mdonovanmsft]

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:

{
  "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

[axisrow]

With Starship/oh-my-posh as the de facto standard for terminal prompt customization, the hardcoded ⎇ glyph stands out as inconsistent. A small "prompt" block in ~/.copilot/config.json for icon overrides would make the CLI feel like a proper citizen of a customized terminal setup. Even just a "disableGlyphs": true escape hatch would go a long way.