Rebased onto #326's session scopes: every shell and page answer, the full
read on open and the background walk write with the scope captured when
their read began, and the store refuses writes from an ended session. The
interim answer-order barrier is gone. Tests cover Supabase signing the
account out, an account switch and a sign-out, on memory and Hive, and the
same account signing back in.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>