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_test.go

@@ -0,0 +1,30 @@
+package stats
+
+import "testing"
+
+func TestStubSystemCPUPercentFirstSampleZero(t *testing.T) {
+	r := NewReporter()
+	if got := r.stubSystemCPUPercent(); got != 0 {
+		t.Fatalf("first sample want 0 got %v", got)
+	}
+}
+
+func TestStubSystemCPUPercentFallbackBelowIdleThreshold(t *testing.T) {
+	r := NewReporter()
+	_ = r.stubSystemCPUPercent()
+	got := r.stubSystemCPUPercent()
+	if got != StubFallbackCPUPercent {
+		t.Fatalf("fallback want %v got %v", StubFallbackCPUPercent, got)
+	}
+	if got >= 20 {
+		t.Fatalf("fallback %v must stay below default idle threshold 20", got)
+	}
+}
+
+func TestStubSystemCPUPercentSubsequentNonZero(t *testing.T) {
+	r := NewReporter()
+	_ = r.stubSystemCPUPercent()
+	if got := r.stubSystemCPUPercent(); got <= 0 {
+		t.Fatalf("subsequent stub sample must be >0 for idle guard, got %v", got)
+	}
+}