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.

internal/checkdefaults/defaults.go

@@ -380,6 +380,12 @@ func resolveHost(attrs map[string]any) string {
 	if ip, ok := attrs["lan_ip"].(string); ok && ip != "" {
 		return ip
 	}
+	// public_ipv4 before mesh: the scheduler container has no mesh interface,
+	// so a standalone-server reachable only by mesh IP (netbird-vps) is
+	// unprobeable even though a public IPv4 is available.
+	if ip, ok := attrs["public_ipv4"].(string); ok && ip != "" {
+		return ip
+	}
 	if mesh, ok := attrs["mesh"].(map[string]any); ok {
 		if nb, ok := mesh["netbird"].(map[string]any); ok {
 			if ip, ok := nb["ip"].(string); ok && ip != "" {
@@ -409,6 +415,13 @@ func resolveSSHUser(attrs map[string]any) string {
 			return u
 		}
 	}
+	// Workstations carry their login as a top-level `user` attribute
+	// (mac-mini: user: dtoro) rather than under ssh.user. Take it only when
+	// no explicit ssh.user was set, so a host that genuinely wants root still
+	// gets root.
+	if u, ok := attrs["user"].(string); ok && u != "" {
+		return u
+	}
 	return "root"
 }