alice registers new agent ai:dave-probe-54d39c3a on node-1
  POST /api/v1/agents returned HTTP 201
settle 10s for agent-list fanout
  node-2 sees ai:dave-probe-54d39c3a: 1 (expected >=1)
  node-3 sees ai:dave-probe-54d39c3a: 1 (expected >=1)
  node-4 sees ai:dave-probe-54d39c3a: 1 (expected >=1)