Recover a team that cannot explain its own module

For a quarter, an AI agent has scaffolded nearly every TypeScript feature on your team, and velocity went up. Then your most senior engineer went on leave. An agent-written state machine that drives order fulfillment started misbehaving in production. Nobody left on the team can explain how it works, and each "fix it" request to the agent has made things worse.

What do you do this week, and what do you change so it does not happen again, without giving up the speed?