Rename source-side log event from 'exported' to 'assigned'
Matches the command rename (tsk export → tsk assign) and the
`assigned=[[ns/tsk-N]]` property name. The destination side keeps its
`accepted` event on accept; both sides now produce a clearly-named
event when a cross-namespace assignment happens.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>