feat: add headless mode support for SSH -N connections

- use s.lifecycle.GetConnection().Wait() to block until SSH connection closes
- Prevent premature session closure in headless mode

In headless mode (ssh -N), there's no channel interaction to block on,
so the session would immediately return and close. Now blocking on
conn.Wait() keeps the session alive until the client disconnects.

server/server.go

@@ -90,7 +90,6 @@ func (s *Server) handleConnection(conn net.Conn) {
 		user = u
 		cancel()
 	}
-
 	log.Println("SSH connection established:", sshConn.User())
 	sshSession := session.New(sshConn, forwardingReqs, chans, s.sessionRegistry, user)
 	err = sshSession.Start()