vms: add monitoring stack and node-exporter for docker host

utility-101-shadow:
- Add full monitoring stack (Prometheus + Blackbox Exporter + Alertmanager)
  with Docker Compose and a systemd unit (monitoring.service)
- Prometheus scrapes: itself, blackbox-exporter, and node-exporter on
  the docker host (docker:9100); blackbox probes cover HTTPS endpoints
  with TLS cert monitoring
- Alertmanager routes warnings to Slack/Discord, critical alerts also
  to email (Gmail SMTP); inhibit rule suppresses SSLCertExpiringSoon
  when SSLCertExpired already fires
- Alert rules: 11 node-exporter alerts (host down, CPU, memory, disk
  fill/prediction, iowait, OOM kill, systemd failed units) + 3 blackbox
  alerts (probe failed, SSL expiring, SSL expired)
- readme: add services list and Docker Engine installation steps

docker host:
- Add node-exporter container running with host pid/network and
  read-only mounts of /proc, /sys, / for full host metrics visibility
- Enable --collector.systemd for systemd unit state metrics
- Add systemd unit (node-exporter.service) to manage the container

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

vms/utility-101-shadow/docker/monitoring/monitoring.service

@@ -0,0 +1,15 @@
+[Unit]
+Description=Monitoring Stack (Prometheus + Blackbox Exporter + Alertmanager)
+After=docker.service network-online.target
+Requires=docker.service
+
+[Service]
+Type=oneshot
+RemainAfterExit=yes
+WorkingDirectory=/srv/docker/monitoring
+ExecStart=/usr/bin/docker compose up -d --remove-orphans
+ExecStop=/usr/bin/docker compose down
+TimeoutStartSec=300
+
+[Install]
+WantedBy=multi-user.target