runtime: enable BIRD BGP on flock-labeled nodes

Calico fenced off via Tigera Installation CR (apps@2121892). flock-agent
now renders bird.conf with the per-node BGP peers; bird sidecar reloads
on changes (debounced 500ms). Re-render tick every 15s reacts to
NodeConfig updates.

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

pkg/agent/runtime_linux.go

@@ -65,22 +65,29 @@ func (s *Server) configureRuntime(ctx context.Context) error {
 	if err := bird.SummaryRoutes(nc); err != nil {
 		s.Logger.Warn("install summary routes", "err", err)
 	}
-	// BGP is intentionally NOT rendered on the first cutover.
-	//
-	// Calico's calico-node DaemonSet still runs on this node (it's
-	// Tigera-Operator-managed via ArgoCD with selfHeal=true) and Calico's
-	// bird is bound to BGP port 179 with the same ASN we'd advertise from.
-	// A clean coexistence requires either an Installation-CR change or a
-	// post-cutover Calico stop. Both are out of scope for the first M2
-	// cutover. crt001 carries a static route for the flock /64 instead.
-	//
-	// To switch to live BGP later: replace this block with bird.Render(nc,
-	// ...) + 15s tick re-render, after disabling calico-node on flock-
-	// labeled nodes. The bird sidecar is already running with a bootstrap
-	// config (just protocol kernel + device — no BGP), so flipping this on
-	// is a one-line change here.
-	s.Logger.Info("BIRD BGP disabled for first cutover; static route on crt001 carries flock /64",
-		"node_cidr6", nc.Spec.CIDR6, "node_cidr4", nc.Spec.CIDR4)
+	// Calico is fenced off this node (Tigera Installation CR adds a
+	// nodeAffinity excluding flock.fritzlab.net/agent on
+	// calicoNodeDaemonSet). flock now owns BGP from this host.
+	if err := bird.Render(nc, nil, nil, routerIDFromNodeIP(s.restCfg)); err != nil {
+		s.Logger.Warn("initial bird render", "err", err)
+	}
+	go func() {
+		t := time.NewTicker(15 * time.Second)
+		defer t.Stop()
+		for {
+			select {
+			case <-ctx.Done():
+				return
+			case <-t.C:
+				cur := s.NodeConfig.Load()
+				if cur == nil {
+					continue
+				}
+				_ = bird.SummaryRoutes(cur)
+				_ = bird.Render(cur, nil, nil, routerIDFromNodeIP(s.restCfg))
+			}
+		}
+	}()
 
 	handler := &PodHandler{
 		Node:         s.Node,