feat(remote): route LXC/VM checks through the Proxmox host, not direct SSH

The scheduler SSHed each guest directly and assumed a deployed probe script
plus working root SSH at the guest's address — false for headless (nfs-export),
keyless (teddycloud), mesh-only (rclone), and macOS (mac-mini) targets, which
left 49 enabled checks stuck "down" on a healthy fleet.

Extract the MCP run tool's resolveExecTarget into a shared internal/remote
package and make it the single execution path for both the scheduler and MCP.
LXC/VM checks now host-hop via pct exec / qm guest exec through the owning
Proxmox host (no per-guest lan_ip, sshd, or authorized key needed); hosts and
workstations resolve their address and user live, so mac-mini's `user: dtoro`
is honored without a re-seed. Address preference now prefers public_ipv4 over
mesh, so netbird-vps is probeable from the scheduler container.

cpu_check.sh gains a real Darwin branch (it reported cpu_pct 0 before).
checkdefaults.resolveSSHUser reads the top-level `user` attribute too.
A machine-target resolution failure is now logged before falling back to baked
config, so a broken probe-config is distinguishable from a real outage.

checks/cpu_check.sh

@@ -2,12 +2,27 @@
 # 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+$3)*100/cores}' /proc/loadavg 2>/dev/null || echo "0")
+os=$(uname -s)
+
+if [ "$os" = "Darwin" ]; then
+  # `top -l 1 -n 0` prints "CPU usage: X% user, Y% sys, Z% idle".
+  # Usage is 100 minus the idle figure that precedes the literal `idle`.
+  USAGE=$(top -l 1 -n 0 -s 0 2>/dev/null | awk '
+    /^CPU usage/ {
+      for (i = 1; i <= NF; i++) {
+        if ($i == "idle") { gsub(/%/, "", $(i - 1)); printf "%.1f", 100 - $(i - 1) }
+      }
+    }' || true)
+else
+  USAGE=$(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
 fi
 
+[ -z "$USAGE" ] && USAGE=0
+
 TEMP=""
 if [ -f /sys/class/thermal/thermal_zone0/temp ]; then
   TEMP=$(awk '{printf "%.1f", $1/1000}' /sys/class/thermal/thermal_zone0/temp 2>/dev/null || true)