Drop the tug alias plus its closest_bookmark and tug_target revsets in favour
of the built-in jj b advance. The bookmark-advance-from default already matches
the old closest_bookmark, and setting revsets.bookmark-advance-to keeps the
empty-scratchpad behaviour (land on @, or @- when @ is empty).
When @ is a fresh empty scratchpad, tug now moves the closest bookmark to
@- so it lands on the actual work instead of the empty change. Driven by a
new tug_target() revset.
