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/install.sh

@@ -0,0 +1,29 @@
+#!/usr/bin/env bash
+# install all check scripts into the canonical directory.
+# Auto-setup hook called by tools/post-pull.sh.
+set -euo pipefail
+
+CLONE_DIR="${HOMELAB_CONTEXT_DIR:-/opt/homelab}"
+CHECK_SRC="$CLONE_DIR/checks"
+CHECK_DST="${OIKOS_CHECK_DIR:-/opt/oikos/checks}"
+
+if [ ! -d "$CHECK_SRC" ]; then
+  exit 0
+fi
+
+mkdir -p "$CHECK_DST"
+
+for script in "$CHECK_SRC"/*.sh; do
+  name=$(basename "$script")
+  if [ "$name" = "install.sh" ]; then continue; fi
+  if [ -f "$CHECK_DST/$name" ]; then
+    if cmp -s "$script" "$CHECK_DST/$name"; then
+      continue
+    fi
+  fi
+  cp "$script" "$CHECK_DST/$name"
+  chmod 755 "$CHECK_DST/$name"
+  echo "[setup-checks] installed $name"
+done
+
+echo "[setup-checks] done"