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.

docs/E2E_VALIDATION.md

@@ -306,11 +306,38 @@ Get-Content ".\data-e2e\logs\<agent-id>.log" -Tail 100
 | Defender off | ✅ | N/A (returns error) | N/A |
 | systemd / LaunchAgent persistence | — | ✅ | ✅ |
 | Idle-mode mining (CPU % sample) | ✅ | ✅ (fixed: /proc/stat) | ✅ (sysctl kern.cp_time) |
+| Headless Linux screenshot | N/A | ✅ with Xvfb — see below | N/A |
 
 Use **Tier 3 Windows VM** when validating spread, GPU, screenshot, or aggressive ops. Use **Tier 2 Linux** for faster C2 regression on recon + mining.
 
 ---
 
+## Linux headless screenshot
+
+Docker/CI/VPS agents have no real display. `scrot`, `import`, and `gnome-screenshot` need an X server.
+
+**Install and start a virtual framebuffer:**
+
+```bash
+apt-get install -y xvfb scrot
+Xvfb :99 -screen 0 1280x720x24 &
+export DISPLAY=:99
+```
+
+Then run the Crucible **Screenshot** action as usual (agent picks up `scrot` on PATH).
+
+**One-shot custom command** (Crucible screenshot command field — no persistent `DISPLAY` in the agent process):
+
+```bash
+DISPLAY=:99 Xvfb :99 -screen 0 1280x720x24 & sleep 1; scrot -q 55 /tmp/s.jpg && base64 -w0 /tmp/s.jpg
+```
+
+On Alpine or minimal images, use the agent forge **custom screenshot command** field with the same pattern.
+
+**Unsupported platforms** (`freebsd`, etc.) return `screenshot not supported on this platform` from `screenshot_stub.go`.
+
+---
+
 ## References
 
 - Automated test phases: `tests/README.md`