Terminate owned subprocess trees

docs/internal/adapters.md

@@ -56,6 +56,12 @@ configured filesystem secrets before adapter initialization.
 - Constructor errors fail stage execution setup early.
 - Runtime adapter errors propagate to stage code and then manifest failure handling.
 - Subprocess adapters persist stage logs/generated configs through stage-managed paths.
+- Shared subprocess execution starts an owned process group on Linux/macOS or a
+  kill-on-close job object on Windows. Cancellation and deadlines request
+  termination, use a bounded forceful fallback, and wait for the leader before
+  returning. Unlogged stdout/stderr use direct null-device descriptors so a
+  descendant cannot retain an adapter pipe after its leader exits. Unsupported
+  platforms reject owned command execution.
 
 ## Implementation And Tests