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.

Empieza rastreando la representación de la identidad en el pie de página de la CLI y la salida de /user; después, inspecciona cómo se resuelven las identidades de GitHub.com, de los hosts Enterprise y de gh. El trabajo estará terminado cuando una opción o configuración del pie de página pueda mostrar el nombre de usuario seleccionado con su nombre de host resuelto en los tres casos, incluidas las identidades via gh.