agent: maintain NetworkUnavailable=False on owned nodes

When Calico shuts down on a flock-labeled node, calico-node sets
NetworkUnavailable=True with reason CalicoIsDown. Nothing replaces it,
so kubelet's NodeController applies node.kubernetes.io/network-
unavailable:NoSchedule and new pods can't land.

flock-agent now patches Status.Conditions every 60s with
NetworkUnavailable=False (reason=FlockReady). RBAC: nodes/status patch.

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

pkg/agent/runtime_linux.go

@@ -53,6 +53,12 @@ func (s *Server) configureRuntime(ctx context.Context) error {
 		return fmt.Errorf("pod informer: %w", err)
 	}
 
+	// Keep NetworkUnavailable=False so the node.kubernetes.io/network-
+	// unavailable taint never gets re-applied. Calico's calico-node sets
+	// it on shutdown; without an owner replacing it, kubelet's controller
+	// taints the node and blocks scheduling.
+	go keepNetworkAvailable(ctx, s.restCfg, s.Node, s.Logger)
+
 	bird := &BirdManager{
 		NodeName:    s.Node,
 		ConfigPath:  "/etc/flock/bird/bird.conf",