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.

Start by tracing the CLI footer identity rendering and the /user output, then inspect how GitHub.com, Enterprise hosts, and gh identities are resolved. Done means an option or footer setting can show the selected username with its resolved hostname in all three cases, including via gh identities.