settle child agents before closing sessions at wind-down

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>

pyproject.toml

@@ -1,6 +1,6 @@
 [project]
 name = "strix-agent"
-version = "1.5.2"
+version = "1.5.1"
 description = "Open-source AI Hackers for your apps"
 readme = "README.md"
 license = "Apache-2.0"

uv.lock

@@ -2378,7 +2378,7 @@ wheels = [
 
 [[package]]
 name = "strix-agent"
-version = "1.5.2"
+version = "1.5.1"
 source = { editable = "." }
 dependencies = [
     { name = "caido-sdk-client" },