Add multi-address fallback for node connectivity

NodeConfig and MasterNodeConfig gain an optional addresses[] field
for fallback addresses tried in order after the primary address.
Provides resilience when Tailscale DNS is down or a node is only
reachable via LAN.

- dialAgentMulti: tries each address with a 3s health check, returns
  first success
- forEachNode: uses multi-address dialing
- AgentPool.AddNodeMulti: master tries all addresses when connecting
- AllAddresses(): deduplicates primary + fallback addresses

Config example:
  [[nodes]]
  name = "rift"
  address = "rift.scylla-hammerhead.ts.net:9444"
  addresses = ["100.95.252.120:9444", "192.168.88.181:9444"]

Existing configs without addresses[] work unchanged.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/master/agentclient.go

@@ -140,21 +140,30 @@ func NewAgentPool(caCertPath, token string) *AgentPool {
 
 // AddNode dials an agent and adds it to the pool.
 func (p *AgentPool) AddNode(name, address string) error {
-	client, err := DialAgent(address, p.caCert, p.token)
-	if err != nil {
-		return fmt.Errorf("add node %s: %w", name, err)
-	}
-	client.Node = name
+	return p.AddNodeMulti(name, []string{address})
+}
 
-	p.mu.Lock()
-	defer p.mu.Unlock()
+// AddNodeMulti tries each address in order and adds the first successful
+// connection to the pool.
+func (p *AgentPool) AddNodeMulti(name string, addresses []string) error {
+	var lastErr error
+	for _, addr := range addresses {
+		client, err := DialAgent(addr, p.caCert, p.token)
+		if err != nil {
+			lastErr = fmt.Errorf("%s: %w", addr, err)
+			continue
+		}
+		client.Node = name
 
-	// Close existing connection if re-adding.
-	if old, ok := p.clients[name]; ok {
-		_ = old.Close()
+		p.mu.Lock()
+		if old, ok := p.clients[name]; ok {
+			_ = old.Close()
+		}
+		p.clients[name] = client
+		p.mu.Unlock()
+		return nil
 	}
-	p.clients[name] = client
-	return nil
+	return fmt.Errorf("add node %s: all addresses failed: %w", name, lastErr)
 }
 
 // Get returns the agent client for a node.