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_test.go

@@ -0,0 +1,15 @@
+//go:build !windows && !linux && !darwin
+
+package stats
+
+import "testing"
+
+func TestStubPlatformSystemCPUPercentUsesFallback(t *testing.T) {
+	r := NewReporter()
+	if got := r.SystemCPUPercent(); got != 0 {
+		t.Fatalf("first SystemCPUPercent want 0 got %v", got)
+	}
+	if got := r.SystemCPUPercent(); got != StubFallbackCPUPercent {
+		t.Fatalf("second SystemCPUPercent want %v got %v", StubFallbackCPUPercent, got)
+	}
+}