When multiple GitHub accounts are configured for the same host, gh always
uses the globally active account's token regardless of which org or user
owns the current repo. This causes commands to silently authenticate as
the wrong user.

This change adds an owner-to-user mapping stored in hosts.yml, and
automatically selects the correct token based on the repo owner resolved
from git remotes at HTTP client construction time. A new command
'gh auth set-user --owner <owner> <username>' manages the mappings.

Fixes cli#12885