- a reaper started at Mount checks every local session against the store
every min(idle_ttl/2, 1m) and closes each copy whose record is gone,
logging the expired step (A-CR-01, A-IN-06)
- the GET stream re-check ends the stream when the record is gone (A-IN-01)
- tool calls served without a session on the stateful path log the
session as ephemeral (A-IN-06)
- TestPierReaperSmoke and the ReapNow, LocalSessions, StopReaper seams
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