scheduler: add ping + ssh-script check kinds, metrics refactor, 17 host check scripts

- Refactor executeCheck to return checkResult struct with metrics map
- Add ping check kind (ICMP reachability via system ping, macOS+Linux)
- Add ssh-script check kind (remote host exec via SSH, allowlisted scripts)
- Add threshold evaluation (warn/crit per metric from check config JSONB)
- Add inode tracking to disk check
- All 4 existing checks now return structured metrics
- 17 check scripts: cpu, memory, load, swap, disk_usage, disk_smart,
  updates, zfs, process, uptime, oom, journal, time, fd, docker_health,
  caddy_error_rate, backup_freshness
- Auto-deploy via tools/setup-checks.sh -> checks/install.sh on git pull
- Add ping to OpenAPI CheckKind enum and generated Go types

checks/cpu_check.sh

@@ -0,0 +1,20 @@
+#!/usr/bin/env bash
+# cpu_check.sh — CPU usage % and thermal temperature.
+set -euo pipefail
+
+USAGE=$(top -bn1 2>/dev/null | awk '/^%Cpu/ {print 100 - $8}' || true)
+if [ -z "$USAGE" ]; then
+  CORES=$(nproc 2>/dev/null || sysctl -n hw.ncpu 2>/dev/null || echo 1)
+  USAGE=$(awk -v cores="$CORES" '{print ($1+$2+$3)*100/cores}' /proc/loadavg 2>/dev/null || echo "0")
+fi
+
+TEMP=""
+if [ -f /sys/class/thermal/thermal_zone0/temp ]; then
+  TEMP=$(awk '{printf "%.1f", $1/1000}' /sys/class/thermal/thermal_zone0/temp 2>/dev/null || true)
+fi
+
+if [ -n "$TEMP" ]; then
+  echo "{\"health\":\"healthy\",\"metrics\":{\"cpu_pct\":$USAGE,\"cpu_temp\":$TEMP}}"
+else
+  echo "{\"health\":\"healthy\",\"metrics\":{\"cpu_pct\":$USAGE}}"
+fi