research coverage

Status of every sub-question (answered / partial / gap). Gaps get one narrower follow-up brief per research round until max_rounds (1-6; default 2 = one follow-up); after that they are disclosed as open rather than widened. With return_pending (a host that runs the briefs itself, and may ask again), a brief planned for the sub-question but not yet run is returned again rather than a follow-up plan