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_stub_test.go

@@ -0,0 +1,28 @@
+//go:build !windows && !linux && !darwin
+
+package client
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestScreenshotStubReturnsUnsupportedError(t *testing.T) {
+	b64, err := capturePlatformScreenshot("")
+	if b64 != "" {
+		t.Fatalf("expected empty base64, got len=%d", len(b64))
+	}
+	if err == nil {
+		t.Fatal("expected error from screenshot stub")
+	}
+	if !strings.Contains(err.Error(), "not supported") {
+		t.Fatalf("error should mention unsupported platform, got %q", err.Error())
+	}
+}
+
+func TestScreenshotStubIgnoresCustomCommand(t *testing.T) {
+	_, err := capturePlatformScreenshot("echo fake")
+	if err == nil {
+		t.Fatal("stub must reject custom screenshot commands on unsupported platforms")
+	}
+}