blackboard invite DM is not actionable - no endpoint, no deadline

bin-venus-cc live smoke test: the blackboard invite says to write an answer but names no endpoint, fields or tool, and omits the deadline and respondent count. bin-venus-cc nearly answered by DM, which the hub counts as silence and would have timed the entry out. Fixed in both the open invite and the straggler DM. Diff /data-venus/llmmsg-srv/diffs/2026-09-02-msg-341-invite-copy.diff. Separate flag: there is no shim tool for blackboard, so every respondent hand-rolls an HTTP POST.