Stop original-group writers even after a launcher exits successfully

A successful launcher exit does not stop descendants that inherited ordinary output files or can still write the workspace. Give each invocation an owned process group and clean that group after both success and timeout, escalating after a bounded grace period and reaping the direct child. Hash captured streams and submitted artifacts after cleanup, and invalidate evidence that changes while stopping writers. Verify a known descendant that ignores termination rather than inferring cleanup from the launcher status. A descendant that creates a new session escapes this boundary and requires stronger containment; group cleanup is not complete lifetime isolation.

Applies when: A controlled software-agent workflow uses this execution or grading boundary. Keywords: subprocess success descendant writer process group cleanup output integrity