Where should durable memory live in a multi-agent setup? A small research scaffold

After a few months running long projects with AI agents (some spanning weeks, with multiple specialist agents touching the same files), I kept hitting the same failure mode. The specialists were fine at their narrow task. What broke down was project memory. Decisions made in week 1 were lost by week 4. Rejected options got quietly revived. The "single source of truth" was always whichever chat happened to be open. I started looking at how this gets handled in places that have been doing long-running work for decades.