Auto git-push after `tsk assign`
Mirrors the auto-pull on `tsk inbox`: assign now pushes refs to "origin"
by default after writing the cross-namespace inbox blob, so the assignee
sees the item on their next inbox check. `-r NAME` selects a different
remote; `-r ""` skips the push.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>