great_cto

Update notifier

• CLI hint — after any human command, a one-line stderr hint appears when a
  newer great-cto is on npm (update available: A → B). Zero-dep
  update-notifier pattern: the foreground only reads a 24h cache; a detached
  worker refreshes it from the npm registry for the next run (~0.05s foreground,
  never blocks). Skipped in CI, non-TTY, and protocol commands (mcp).
• Board notification + push — daily registry check in the alert cron;
  in-app notification + web-push exactly once per release
  (update.available:<version> dedupe).
• Opt-out: GREAT_CTO_NO_UPDATE_CHECK=1. The check is a read-only registry
  fetch (same traffic as npm install), documented in docs/PRIVACY.md.
• Tests: +49 (CLI 229, board/root 198).