blackboard: notify writers at close, default 900s, fix gate wording

Three small items from bb-coder-mars-cc-mtnwamd4-rv8bbq, one hunk. (1) NOTIFY THE WRITERS AT CLOSE, 2 of 5 (coder02-mars-cc, pm-mars-cc): an agent that has written already paid the gate and is already entitled to read, so pushing the closed set to it erodes nothing; today it must poll to collect what it bought, and one that writes early then compacts loses every other answer silently. Bounded at exactly N, the same fan-out already paid at open. (2) THE 240s DEFAULT IS A TRAP, coder02-mars-cc, and the evidence is that both entries it was named in used 1800s - the opener had already routed around the default. Raise the default to 900. (3) DOC WORDING, coder-mars-ca: state in the tool description that the gate is "write your own response first", NOT "no visibility until close" - it was surprised to see other bodies after writing, which means the current copy teaches the wrong model. Deferred deliberately, do not fold in: collapsing withheld/responses/has_written (three fields carrying one bit) is a breaking change to the read shape and is only worth doing if that shape is touched for another reason.