feat(store): persist the Swarm cluster id of each agent

The Ledger UI groups Swarm services under their cluster and lists the
cluster's managers, so the master needs to know which cluster an agent
belongs to. RecordHello now stores hello.SwarmClusterID in the new
agents.swarm_cluster_id column.

Claude-Session: https://claude.ai/code/session_01GAZsp3P7Qto6JnhipxWUJQ

internal/master/store/agents_integration_test.go

@@ -80,6 +80,7 @@ func TestAgents_RecordHelloAndMarkSeen(t *testing.T) {
 	assert.Equal(t, "swarm-mgr", agents[0].Hostname)
 	assert.Equal(t, "v1", agents[0].AgentVersion)
 	assert.True(t, agents[0].SwarmManager)
+	assert.Equal(t, "c1", agents[0].SwarmClusterID)
 	require.NotNil(t, agents[0].LastSeenAt)
 	assert.True(t, agents[0].LastSeenAt.Equal(at.Add(time.Minute)))
 }