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/model/model.go

@@ -27,9 +27,12 @@ type Agent struct {
 	Hostname     string
 	AgentVersion string
 	SwarmManager bool
-	LastSeenAt   *time.Time
-	RevokedAt    *time.Time
-	CreatedAt    time.Time
+	// SwarmClusterID is the Swarm cluster the agent's daemon was in when it last
+	// connected; "" outside a swarm.
+	SwarmClusterID string
+	LastSeenAt     *time.Time
+	RevokedAt      *time.Time
+	CreatedAt      time.Time
 }
 
 // Service is one service as last reported by an agent.