S00-014: Make successful runner process-tree cleanup idempotent

Changes: Record completed termination on the Popen instance after all cleanup steps succeed; avoid repeated signals to the reaped group. Add deterministic repeated-EPERM and failed-descendant-kill retry regressions without suppressing permission errors.
Validation: Four focused cleanup tests passed; make check passed 72 tests and plan/snapshot validation. git diff --check passed. Natural prior EPERM did not recur; injected EPERM reproduced the pre-fix failure. No application or parent-task acceptance is claimed.

Task: S00-014
Agent: auto-ci-cd-14343249da62
Role: ci-cd
Action: claim
Status: review

Task title: Fix repeated runner cleanup after a submitted parent is reaped

scripts/runner.py

@@ -335,6 +335,10 @@ The coordinator commits each transition before launching another agent.
 
 
 def stop_process_tree(process):
+    # Transition/deadline handling and finally share this Popen instance. Once
+    # cleanup succeeds, do not signal its reaped (possibly reused) group again.
+    if getattr(process, '_luma_tree_stopped', False) is True:
+        return
     if os.name == 'posix':
         try:
             os.killpg(process.pid, signal.SIGTERM)
@@ -360,6 +364,10 @@ def stop_process_tree(process):
         except ProcessLookupError:
             pass
 
+    # Set only after all termination steps succeed. Permission errors must
+    # propagate, including when the parent exited but a descendant is live.
+    process._luma_tree_stopped = True
+
 
 def task_scope(tracker, task_id):
     return tracker.task(task_id)['scope']