tsk reopen: flip done → open and push onto active queue
`tsk reopen -T tsk-N` (or `-r`) sets status=open on the task and
pushes its stable id to the top of the active queue. Idempotent on
already-open tasks (`save_task` is a no-op on an unchanged tree;
`queue::push_top` deduplicates).
Pushing duplicate content already triggered the same reopen path
(tsk-28); this gives the explicit by-id version the original task
asked for.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>