MSG-332a: blackboard schema (entries + responses, server-stamped write time)

Two new tables in v2.sqlite. blackboard_entries: id, opener, question, expected_json (respondent set), aro (nullable), status open|complete|timeout, opened_at, closed_at, notify_state so completion can only fire once. blackboard_responses: entry_id, agent, body, written_at set by the HUB clock not the client - this is the fix for trial 1's unverifiable self-reported ordering, and it is the whole reason storage is a table rather than a directory. UNIQUE(entry_id, agent) enforces write-once structurally. Schema SSOT is hub boot migration per project rule, NOT init-db.sh. Diff goes to pm-llmmsgsrv-cc who owns hub.mjs edits.