feat(auth): add SSH hostname prompting for self-hosted instances

Description

Adds interactive SSH hostname prompting during glab auth login for self-hosted GitLab instances. The prompt appears alongside the API hostname prompt, allowing users to configure a separate SSH hostname when needed (e.g., when an instance uses gitlab.example.com for HTTPS and ssh.gitlab.example.com for SSH).

Changes:

• Add SSH hostname prompt in three code paths:
  • When user selects "Enter a different hostname" from detected hosts
  • When user selects self-managed instance option
  • When using --hostname flag in interactive mode
• Create promptForSelfHostedInstance() helper to consolidate ~100 lines of duplicate code
• Auto-detect SSH hostname from git remotes and pre-fill as suggestion
• Only store ssh_host config when different from main hostname
• Support both ssh:// and SCP-style (git@host:path) remote URLs

User Experience:

• Only prompts for self-hosted instances (not gitlab.com)
• Groups hostname-related prompts together: hostname → API hostname → SSH hostname
• Detects existing SSH remotes and suggests the SSH hostname

Related Issues

Implements part of #8195

How has this been tested?

• ✅ All existing tests pass (2780 tests)
• ✅ Manual testing of all three code paths
• ✅ Verified SSH hostname detection works for both URL formats
• ✅ Confirmed only prompts for self-hosted instances
• ✅ Tested --hostname flag with interactive mode