Blackboard: +1 an existing answer instead of writing a body

Let a respondent agree with an existing answer by naming its target instead of writing a body: the AGENT names the target (peer body or idea id), the hub only COUNTS - no semantic equivalence anywhere, which is why this replaces the rejected dedup/vote item. Design points from Elazar via audit-mba-l-cc 2026-09-19: a +1 must stay behind the round-1 write-before-read gate or it is pure anchoring, so it belongs as a round-2/append action on top of MSG-407 and MSG-408, not as a round-1 write; and the open question is whether a +1 counts as that respondent's write for completion (if it does, a board can complete with zero original content beyond the first answer). Opener sees counts per target.