When gt down --all killed all Gas Town sessions, if those were the only
tmux sessions, the server would exit due to tmux's default exit-empty
setting. Users perceived this as gt down --all killed my tmux server.
Fix: Set exit-empty off before killing sessions, ensuring the server
stays running for subsequent gt up commands. The --nuke flag still
explicitly kills the server when requested.
Fixes: gt-kh8w47
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>