Validity interval carries event time, not transaction time

Status: accepted (2026-07-06), implementation pending

agent_memory.valid_from/valid_to today duplicate the transaction-time audit stamps (created_at/superseded_at) and are never queried. We decided they are the event-time validity interval: valid_from is when the fact became true in the world (supplied by the caller, e.g. conversation dates), not when the row was inserted. This makes as-of retrieval ("what was true before X?") a queryable column filter instead of regex-parsing dates out of stored text, which is how the benchmarks currently smuggle event time. The rejected alternative — dropping the columns as dead schema — was simpler but forfeits the only temporal-accuracy lever the schema has.