Phase D: Automated DNS registration via MCNS

Add DNSRegistrar that creates/updates/deletes A records in MCNS
during deploy and stop. When a service has routes, the agent ensures
an A record exists in the configured zone pointing to the node's
address. On stop, the record is removed.

- Add MCNSConfig to agent config (server_url, ca_cert, token_path,
  zone, node_addr) with defaults and env overrides
- Add DNSRegistrar (internal/agent/dns.go): REST client for MCNS
  record CRUD, nil-receiver safe
- Wire into deploy flow (EnsureRecord after route registration)
- Wire into stop flow (RemoveRecord before container stop)
- 7 new tests, make all passes with 0 issues

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

internal/agent/deploy.go

@@ -164,6 +164,13 @@ func (a *Agent) deployComponent(ctx context.Context, serviceName string, cs *mcp
 		}
 	}
 
+	// Register DNS record for the service.
+	if a.DNS != nil && len(regRoutes) > 0 {
+		if err := a.DNS.EnsureRecord(ctx, serviceName); err != nil {
+			a.Logger.Warn("failed to register DNS record", "service", serviceName, "err", err)
+		}
+	}
+
 	if err := registry.UpdateComponentState(a.DB, serviceName, compName, "running", "running"); err != nil {
 		a.Logger.Warn("failed to update component state", "service", serviceName, "component", compName, "err", err)
 	}