brainstorm-server-requires-persistent-systemd-not-per-session

The superpowers brainstorming WebSocket server (brainstorm.service, port 17777) was designed session-scoped but works better as a persistent systemd service — this avoids port conflicts across overlapping sessions and eliminates per-session startup latency. Auto-cleanup of stale screen artifacts handled by session-close (>24h) and a daily 04:00 cron (>7d).