Provenance annotation

A URL attached to a backlog line, meaning that this line and that external item are the same work.

It is written immediately before the line's status token, and read by hostname, so several tools can annotate the same entry without colliding. A line can carry more than one.

Its only job is to stop a mirrored task from coming back as a duplicate. Presence says the two records are one piece of work, and nothing else.