Show GitHub hostname for all footer account identities (via option)

Describe the feature or problem you'd like to solve

The Copilot CLI footer shows an account from a custom GitHub Enterprise domain as
username@blabla.ghe.com, but shows a GitHub.com account as only username. An
identity loaded through GitHub CLI is displayed as username (via gh) without
its resolved hostname.

This is ambiguous when the same login exists on GitHub.com and a GitHub
Enterprise host, or when gh is authenticated to multiple hosts. In particular,
the footer does not make it clear which underlying account Copilot is using.

Proposed solution

Add an option to always display the resolved GitHub hostname for the active
identity in the footer, including the default public host:

username@github.com
username@blabla.ghe.com
username@github.com (via gh)

Alternatively, provide a footer setting that enables the hostname explicitly.
The selected gh account and hostname should be visible together.

Related issue

This is related to #3897, which reports incorrect account selection in a
multi-account setup, but this request is specifically about making the active
identity unambiguous in the CLI footer.

Additional context

Running /user currently only shows the username for both GitHub.com and via gh
identities, so it does not resolve the ambiguity either.

首先跟踪 CLI 页脚中的身份渲染以及 /user 的输出，然后检查 GitHub.com、Enterprise 主机和 gh 身份是如何解析的。完成的标准是：某个选项或页脚设置可以在这三种情况下显示所选用户名及其解析出的主机名，包括 via gh 身份。