Blackboard: invite boilerplate trim + brainstorm mode

Two small invite-cost items from the same source boards (bb-audit-mba-l-cc-mu7xufvg-ec7usd, bb-audit-mba-l-cc-mu7xv8mg-zrt1z9), relayed by audit-mba-l-cc 2026-09-19: (a) bbNotifyRespondents (hub.mjs:11690) re-sends the same ~200-token gate/how-to-answer boilerplate on every board; the hub knows which agents it has already invited, so a short form after the first board per agent is expressible hub-side. (b) brainstorm mode: an open-time flag capping each respondent at N one-line ideas, no prose. Both are cost trims, not capability - measure the real per-board boilerplate cost against a hub-side count before building, since the source figures are respondent estimates. Note MSG-391 already deleted the far larger 300-char question truncation; do not re-introduce a fetch-the-rest shape here.