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

planning/stages/S00.md

@@ -18,7 +18,7 @@
 - [S00-011](../tasks/S00-011.md): Restore verified npm registry access for the stack spike — ci-cd; done.
 - [S00-012](../tasks/S00-012.md): Fix runner prompt delivery after partial stdin timeout — ci-cd; blocked.
 - [S00-013](../tasks/S00-013.md): Bound oversized S21-006 runner context without truncating requirements — ci-cd; done.
-- [S00-014](../tasks/S00-014.md): Fix repeated runner cleanup after a submitted parent is reaped — ci-cd; todo.
+- [S00-014](../tasks/S00-014.md): Fix repeated runner cleanup after a submitted parent is reaped — ci-cd; review.
 - [S00-010](../tasks/S00-010.md): Gate: Профиль и проверка стека — qa; todo.
 
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