Fleet monitoring: sysstat 28d + perfsnap/PSI/iotop timers + on-demand toolbox container (#826)

- monitoring/node-install.sh: idempotent per-node installer (sar HISTORY=28,
  three oneshot timers, per-node conf with qemu.slice/lxc cgroup globs)
- monitoring/collectors + systemd units: iotop-snap 15min, perfsnap 60s,
  psi-textfile 15s (now sources /etc/knelperf/perfsnap.conf; multi-glob)
- monitoring/toolbox: pct LXC (vmid 950, privileged, apparmor unconfined,
  stopped at rest) + knelperf-toolbox wrapper + Dockerfile variant
- monitoring/deploy-fleet.sh: workstation-side rollout driver
- piloted on pfv-tsys1: per-VM PSI gauges verified live

Redmine: https://projects.knownelement.com/issues/826

monitoring/systemd/knelperf-iotop-snap.timer

@@ -0,0 +1,10 @@
+[Unit]
+Description=Run knelperf-iotop-snap every 15 minutes
+
+[Timer]
+OnCalendar=*:0/15
+Persistent=true
+RandomizedDelaySec=30
+
+[Install]
+WantedBy=timers.target