async hooks get killed at session teardown, background work never finishes

2.1.220, Windows, headless claude -p.

Stop hook with "async": true that writes a start marker, sleeps 8s, then writes a done marker. Start marker is always there, done marker never shows up - the process gets killed when the session tears down. Well under the 10 minute async budget.

Docs mention the budget but say nothing about teardown killing async hooks. If that's intended, fine, but it should be documented. Right now the only reliable way to run work that outlives the session is spawning a detached process yourself.

Repro is two ~10 line files, can paste if useful.