Automated update check

[MFelida]

Notify users an update is available.
User MUST be able to silence the notifications.

[CheapFuck]

This looks already implemented by towel_auto_update_check() in towel-common (it runs on every towel invocation via bin/towel). Both requirements from the issue are covered:

Notify users an update is available ✅

• Runs a throttled, once-per-day check in the background (non-blocking; results cached in update_check).
• When an update is available it prints to stderr, at most once per day (tracked via update_notified):

  towel update available: <current> -> <latest>
    Run 'towel update --apply' to update, or 'towel update --no-auto-check' to disable these notifications.
  

• Skipped inside containers (CONTAINER_ID set), so it only nags on the host.

User MUST be able to silence the notifications ✅ — three ways:

• TOWEL_NO_UPDATE_CHECK=1 (env var, documented in towel update --help).
• towel update --no-auto-check → creates $TOWEL_DATA/no-update-check; re-enable with towel update --auto-check.
• The notification itself tells the user how to disable it.

Unless something specific is still missing (e.g. a desired notification channel beyond stderr), I think this can be closed. Happy to open a small follow-up if you'd like the silence flags surfaced in the top-level towel --help too.