Fix exotic CPU stub idle-mining guard and document Linux headless screenshots.

Return sane fallback CPU percent after first sample on unknown Unix platforms; add xvfb/scrot docs and stub tests; clear PROBLEMS.md open bugs.

agent/stats/cpu_stub.go

@@ -3,5 +3,5 @@
 package stats
 
 func (r *Reporter) SystemCPUPercent() float64 {
-	return 0
+	return r.stubSystemCPUPercent()
 }