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/client/screenshot_linux_test.go

@@ -0,0 +1,28 @@
+//go:build linux
+
+package client
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestLinuxScreenshotErrorMentionsHeadlessXvfb(t *testing.T) {
+	_, err := captureLinuxScreenshotJPEG()
+	if err == nil {
+		t.Skip("screenshot tool present on this host")
+	}
+	if !strings.Contains(strings.ToLower(err.Error()), "xvfb") {
+		t.Fatalf("headless error should mention xvfb, got %q", err.Error())
+	}
+}
+
+func TestLinuxScreenshotCustomCommandEmptyRejected(t *testing.T) {
+	_, err := capturePlatformScreenshot("   ")
+	if err == nil {
+		t.Skip("default capture succeeded without custom command")
+	}
+	if !strings.Contains(err.Error(), "scrot") && !strings.Contains(err.Error(), "xvfb") {
+		t.Fatalf("expected tool or xvfb hint, got %q", err.Error())
+	}
+}