Allow skipping videos using YouTube video tag `skip-sync`

I don't know if this is already a thing, but if somebody doesn't want to sync a specific video from YouTube, because they might upload some kind of changed version, a tag like `skip-sync` on YouTube that the ytsync will detect and skip would be cool.

Start by tracing ytsync's YouTube synchronization path and how video tags or metadata are exposed. Determine whether the `skip-sync` tag can be detected, then verify that tagged videos are skipped while untagged videos continue syncing.

Written by the indexing model from the issue text.