Add NetBird update scripts and README

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

update-netbird-client.sh

@@ -0,0 +1,16 @@
+#!/usr/bin/env bash
+# Update the NetBird client (apt package) on this host.
+# Warning: restarting the client drops sessions that go over NetBird.
+set -euo pipefail
+
+echo "Current: $(netbird version)"
+
+sudo apt-get update
+sudo apt-get install -y --only-upgrade netbird
+
+sudo systemctl restart netbird
+sleep 3
+
+echo "Now: $(netbird version)"
+systemctl is-active netbird
+netbird status