Verification mode or tool that checks the target URLs

This could check:

1. The URL returned a success code. This seems valuable, but should be off by default since it would require internet connectivity and will definitely slow things down.
2. Maybe provide means to check that the target tag at that URL pointed back to the current tag.
   • This is harder since the URL may not return just the text, or even a tag (it could be documentation).
   • May need to regexp the returned content and see if a corresponding tag exists, then act accordingly.
3. If the target URL is a GitHub repo, we could potentially work out if that file has changed since the commit SHA of the URL, and then warn that there have been updates. This sounds pretty hard too, but not impossible.
4. Could provide a mapping of URL to local repo, and then, for repo URLs, use that to verify the tags.

No files, tests, or entry points are named. Start by reviewing the existing URL and synchronization behavior, then narrow the proposal to one verification mode and define how success, failures, and network access should be handled. Done should include an agreed scope and tests for the selected behavior.