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

state/tasks.json

@@ -1,6 +1,6 @@
 {
   "schema_version": 1,
-  "revision": 19,
+  "revision": 21,
   "tasks": {
     "S00-001": {
       "status": "done",
@@ -1165,10 +1165,14 @@
       "completed_at": "2026-10-14T18:42:03+00:00"
     },
     "S00-014": {
-      "status": "todo",
-      "owner": null,
+      "status": "review",
+      "owner": "auto-ci-cd-14343249da62",
       "external_wait": false,
-      "reason": null
+      "reason": null,
+      "started_at": "2026-10-14T18:45:17+00:00",
+      "updated_at": "2026-10-14T18:47:20+00:00",
+      "evidence": "state/evidence/S00-014-auto-ci-cd-14343249da62.md",
+      "handoff": "state/handoffs/S00-014.md"
     }
   },
   "events": [
@@ -1406,6 +1410,29 @@
       "from": "in_progress",
       "to": "blocked",
       "reason": "Submit-transition descendant regression fails: repeated stop_process_tree after parent reap raises EPERM at os.killpg; launch OSError handling then attempts invalid escalation from review state."
+    },
+    {
+      "revision": 20,
+      "at": "2026-10-14T18:45:17+00:00",
+      "action": "claim",
+      "task": "S00-014",
+      "agent": "auto-ci-cd-14343249da62",
+      "role": "ci-cd",
+      "from": "todo",
+      "to": "in_progress",
+      "reason": null
+    },
+    {
+      "revision": 21,
+      "at": "2026-10-14T18:47:20+00:00",
+      "action": "submit",
+      "task": "S00-014",
+      "agent": "auto-ci-cd-14343249da62",
+      "role": null,
+      "from": "in_progress",
+      "to": "review",
+      "reason": null,
+      "evidence": "state/evidence/S00-014-auto-ci-cd-14343249da62.md"
     }
   ],
   "retired_agents": [