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_common.go

@@ -1,5 +1,21 @@
 package stats
 
+// StubFallbackCPUPercent is returned after the first sample on platforms without
+// OS CPU counters (cpu_stub.go). Kept below the default idle threshold (20%).
+const StubFallbackCPUPercent = 5.0
+
+// stubSystemCPUPercent implements idle-mining-safe defaults when real counters
+// are unavailable: first sample returns 0 (no delta yet), then a low busy %.
+func (r *Reporter) stubSystemCPUPercent() float64 {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	if !r.hasSample {
+		r.hasSample = true
+		return 0
+	}
+	return StubFallbackCPUPercent
+}
+
 func cpuBusyPercentFromDeltas(idleDelta, totalDelta float64) float64 {
 	if totalDelta <= 0 {
 		return 0