cmd/cli: skip upstream.os healthcheck when WFP loopback protect enabled

When WFP loopback protect is active, the upstream.os healthcheck will
always fail because an external WFP block filter is interfering with
plain DNS. This demotes those expected failures to debug level and
returns errOsHealthcheckSuppressed so the recovery loop treats them
as non-fatal, eliminating the log spam described in #526.

cmd/cli/dns_intercept_others.go

@@ -37,3 +37,7 @@ func (p *prog) scheduleDelayedRechecks() {}
 
 // pfInterceptMonitor is a no-op on unsupported platforms.
 func (p *prog) pfInterceptMonitor() {}
+
+// osHealthcheckSuppressed always returns false on non-Windows platforms —
+// WFP loopback protect (the trigger for suppression) is Windows-only.
+func (p *prog) osHealthcheckSuppressed() bool { return false }