Add syscall.Flock() exclusive lock in daemon.Run() to prevent TOCTOU
race condition where concurrent 'gt daemon start' commands could spawn
multiple daemons. Only the first to acquire the lock succeeds; others
exit cleanly. Lock is per-town (in townRoot/daemon/daemon.lock) so
multiple GT instances from different directories work independently.
Also detect race losers in runDaemonStart() by comparing spawned PID
with PID file, reporting 'already running' instead of false success.
c25ff34b5b