The Agent Hub kill path called AgentLifecycleManager.release(), which
disposes the session and then unregisters the ref while leaving the
on-disk <id>.jsonl intact. On the next hub open, registerPersistedSubagents
rescans the transcript tree and its `if (!registry.get(id))` guard cannot
distinguish an explicit kill from a normally-parked agent, so it re-adopts
the killed id as a fresh `parked` row.
Add a `tombstone` option to release() that mirrors finalizeSubagentLifecycle's
genuine-kill path: dispose and detach the session but keep the ref registered
as terminal `aborted` instead of removing it. The kept-registered id makes the
rescan guard skip it, and the transcript stays on disk (still reachable via
history://<id>, per #5261). The hub kill button now passes tombstone: true.
Fixes#7250
roboomp
65fd8cb0e8