blackboard: extend timeout, better length error, 8000-char question cap

From coder-mars-cc's field report 2026-09-05. timeout_secs is fixed at open and cannot be extended, so an entry whose respondents are mid-investigation closes with partial answers and the work already done is wasted. Op: opener-only extend of an entry still status='open', bounded by the existing 3600s ceiling measured from opened_at, refused once the entry has closed. Also in scope, same hunk: bb_question_too_long must report the ACTUAL length and the limit in its message - it currently rejects a whole assembled question with neither, which pushes the opener to cut evidence under time pressure. And raise the question cap from 4000 to 8000 to match blackboard_write's body limit; the asymmetry (an agent can ANSWER at twice the length it can ASK) has no rationale, and the cost lands in blackboard_read, which is on-demand, not in the per-respondent invite, which stays capped at 300 chars.