fix(update-notifier): only mark sentinel checked when notice is shown

[nkolev92]

Summary

The update-notifier sentinel was written as soon as a check started, not when a notice was actually shown. Fast commands (e.g. npm version) wrote the sentinel and exited before the notice could ever print, so the throttle window was wasted and the user never saw the update message.

This changes the sentinel's mtime to mean "last time a notice was shown", and retries the check on every eligible run until a notice is actually displayed.

Changes

• updateCheck() returns { message, onShown } instead of a bare string. The sentinel is only written when onShown() runs, after the notice is logged.
• onShown() uses writeFileSync since it's called from the process.on('exit', ...) handler, where async work can't be awaited.
• Removed the weekly/daily split between GA and prerelease — now a single daily check for both.
• Bundled npm.updateNotification into { message, onShown }, removing the separate updateNotificationShown field.
• Silent runs don't show or record the notice, so a later non-silent run gets another chance.
• Node.js engine compatibility is still evaluated fresh every run, never cached.

Tradeoffs

If a notice is never shown, every later eligible run retries the check. We preferred this over a more complex sentinel that caches a pending version/check state, since that would need to be environment-aware (Node version can change between runs) — not worth the added complexity.