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.go

@@ -1,5 +1,17 @@
 //go:build linux
 
+// Headless Linux hosts (Docker, CI, VPS) have no DISPLAY. Install Xvfb + scrot:
+//
+//	apt-get install -y xvfb scrot
+//	Xvfb :99 -screen 0 1280x720x24 &
+//	export DISPLAY=:99
+//
+// Or pass a one-shot custom command via Crucible screenshot (command field), e.g.:
+//
+//	DISPLAY=:99 scrot -q 55 -o /tmp/s.jpg && base64 -w0 /tmp/s.jpg
+//
+// See docs/E2E_VALIDATION.md § Linux headless screenshot.
+
 package client
 
 import (
@@ -76,5 +88,5 @@ func captureLinuxScreenshotJPEG() ([]byte, error) {
 		return os.ReadFile(path)
 	}
 
-	return nil, fmt.Errorf("no screenshot tool found (scrot, import, gnome-screenshot)")
+	return nil, fmt.Errorf("no screenshot tool found (scrot, import, gnome-screenshot); headless hosts need xvfb — see docs/E2E_VALIDATION.md")
 }