Map ssh_host to API host when resolving git remotes

[andrew]

Closes #41.

Self-hosted GitLab can serve git-over-ssh on a different host than the web UI and API. A remote like git@ssh.gitlab.test:owner/repo.git parses to domain ssh.gitlab.test, which then fails forge detection or hits the wrong API endpoint.

This adds an ssh_host key to the per-domain config:

[gitlab.test]
type = gitlab
ssh_host = ssh.gitlab.test

When repoFromGitRemote parses a remote URL, it checks whether the extracted domain matches any configured ssh_host and swaps in the section name (the API host) before building the client. The reverse lookup lives on *Config as DomainForSSHHost.

Without the config the behaviour is unchanged — domains pass through. The mapping only kicks in when explicitly declared.

This is the same approach glab takes (https://gitlab.com/gitlab-org/cli/-/merge_requests/2924).