Fix universe concierge notify handoff

[asim]

Summary

• Route the universe harness notify stage through an explicit concierge Agent.Chat request that tells the agent to call notify.Send for buyer@acme.com before replying.
• Keep the existing side-effect assertion so the flow only completes after the buyer notification is observed.

Closes #3804
Closes #3810

Testing

• go test ./internal/harness/universe -run 'TestUniverseHarnessContract|TestNotify' -count=1\n- go build ./...\n- go test ./... (fails in this sandbox because loopback gRPC targets are blocked by envoy for client/grpc and transport/grpc)\n- golangci-lint run ./...