Every error path cancels the agent subtree before returning, so children unwind
while their sessions are still open. The success path did not: when root called
finish_scan with children still mid-turn, the finally closed their sessions
underneath them, the in-flight save raised "SQLiteSession is closed", and those
children were marked crashed - failing a scan whose report was already written.
Cancelling the descendants moves into the finally, ahead of the session close,
so it runs on every path - clean finish included. The per-branch cancels are
dropped since the finally now covers them; each branch keeps only its own root
status. cancel_descendants targets child tasks (root runs inline with no task),
and the finally already awaits snapshot and cleanup under cancellation, so the
added await is safe there.
Fixes#1012.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
