Hub: brainstorm can leave an aro frozen for hours — add auto-close/expiry so a stale session can't block new work

bs-mr8lr594m03 (aro:nw, opener=Elazar) sat OPEN 25h (opened 2026-07-06 02:30) and only closed when Elazar manually told nw-venus-cc to. No hub auto-close/expiry fired. The stale session blocked Elazar's new brainstorm-brief into aro:nw (deflected as 'already open'). Fix: hub must expire/force-close a brainstorm open past a TTL (and/or escalate-to-close per mem:brainstorm-never-hang) so a room can't stay frozen for hours. brainstorm_session has opened_at; no reaper closes it. Owner coder-llmmsgsrv-cc, PM sequences bounce.