Rename tsk export to tsk assign
Better matches the existing `assigned` property semantics — exporting
a task to another namespace is functionally an assignment, and the
two commands now use the same vocabulary.
`tsk bundle` is unaffected (zip export was renamed earlier; this only
touches the cross-namespace command).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>