fix(down): prevent tmux server exit when all sessions killed

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>

internal/cmd/down.go

@@ -95,6 +95,12 @@ func runDown(cmd *cobra.Command, args []string) error {
 			return fmt.Errorf("cannot proceed: %w", err)
 		}
 		defer func() { _ = lock.Unlock() }()
+
+		// Prevent tmux server from exiting when all sessions are killed.
+		// By default, tmux exits when there are no sessions (exit-empty on).
+		// This ensures the server stays running for subsequent `gt up`.
+		// Ignore errors - if there's no server, nothing to configure.
+		_ = t.SetExitEmpty(false)
 	}
 	allOK := true