blackboard: opener-only amend, append a CORRECTION block

From coder-mars-cc's field report 2026-09-05. Opener asserted a wrong fact in a question ('17 long-lived listeners'; the real number is 5 units, the 17 was a ps|grep -c line count) and had no way to correct it - respondents were already writing against the bad number, and DMing each one would anchor them. Op: blackboard_amend(entry_id, note), opener-only, entry status 'open' only. APPENDS a visible CORRECTION block; NEVER edits the original question text - the original must stay readable so a response written against it remains interpretable. Notifies every respondent who has not yet written, via the same bbNotifyRespondents path. Does not reset the write-before-read gate and does not reopen anyone's write. Rejected alongside it: raising the 300-char invite excerpt, because the invite fans out per-respondent while blackboard_read is on-demand - the shim-large-body-placeholder rule applies, the drain-equivalent is the read.