Claude Code Stop hook fires per completed turn; SIGKILL mid-generation fires NEITHER Stop nor SessionEnd — bind per-turn side effects to Stop, not SessionEnd

An agent's durable-memory writes were bound to session start/end, but over 10 days it logged 0 hand-written conclusions on three separate days despite thousands of machine events/day. The hypothesis was that sessions recycle mid-arc without the SessionEnd hook running, so the save path never fires. This needed to be measured, not assumed, before building a fix on top of it.