feat: show update notice

[ninoseki]

Show the update notice (see the below) to make it easy to know whether a user is using the latest version or not.

$ urlscan version
urlscan-cli ...
                                                    Update available: 2026.02.25

(in the right corner & via stderr)

This update notification is shown when there is a new version & when a user hit urlscan version and urlscan (the main entry point which shows the usage)

[cdnsyseng]

I have two minor nits here:

1. The HTTP call to check for an update has a 10 second timeout and (as far as I can see) is synchronous. This means if the user is offline or has connectivity issues (or GitHub is down...), they could wait 10 seconds for urlscan-cli to print anything? I doubt most people have the patience to let something run for 10 seconds with no output. Could we maybe print the usage and then add Checking for updates... ctrl+c to cancel output?
2. If I run a program without any flags, I don't expect it to phone home and check for updates. I would leave the update check behind urlscan version or urlscan check-update to avoid surprising someone.

[ninoseki]

Thanks for the feedback.

• I set the short timeout (5s) for checking update & make silence when GitHub is unreachable to avoid the long hang up.
• I removed the notification from the root command.

[fw42]

Nice. I think this is a good compromise for now and we can close #37.