macOS onboarding requires GitHub sign-in and hides the BYOK-only path

### Short summary

The macOS first-run onboarding requires GitHub authentication and does not expose the intended option to start using BYOK without signing in.

### Affected version or release

GitHub Copilot app 1.1.11

### Installation context

Local desktop installation on macOS 26.6.1 (Build 25G76); first-run onboarding before any organization or repository is selected.

### What happened?

When launching the GitHub Copilot app while signed out, the first screen presents only these actions:

- Sign in to GitHub
- Sign in to GitHub Enterprise Cloud (*.ghe.com)

There is no visible way to skip authentication, continue to BYOK setup, or use a local/external model provider without first signing in to GitHub.

This conflicts with the intended onboarding behaviour described by Pierce Boggan: users should be able to start using BYOK without logging in.

### Steps to reproduce

1. Install or open GitHub Copilot app 1.1.11 on macOS.
2. Ensure there is no authenticated GitHub session in the app.
3. Launch the app.
4. Observe the first onboarding screen.

### Expected behavior

The onboarding flow should provide a visible option to skip GitHub authentication and proceed directly to BYOK/provider configuration. And use the GitHub Copilot app without having a github account. For non-github repositories

### Additional context

Related conversation:

- [Reporter's original screenshot post](https://x.com/jitendravyas/status/2090522508596576311)
- [Expected BYOK-without-login behaviour](https://x.com/pierceboggan/status/2090520156661579959)
- [Request to log this bug](https://x.com/pierceboggan/status/2090561921812689073)

复现 issue 中描述的 macOS 未登录状态下的首次运行引导流程，并追踪仅呈现 GitHub 登录选项的入口点；未指定源文件或测试。完成标准是用户可以跳过身份验证，进入 BYOK/提供商配置，并在没有 GitHub 账户的情况下将应用用于非 GitHub 仓库。

由索引模型根据 Issue 内容生成。