feat(mcp-image-gen): add ComfyUI auto-start health check + systemd service

Option A: Add lifespan context manager to server.py
- _ping_comfyui(): async health check against /system_stats
- check_and_start_comfyui(): ping on startup; if down, launches ComfyUI
  via subprocess.Popen from COMFYUI_DIR (.venv/bin/python main.py)
  with HSA_OVERRIDE_GFX_VERSION=11.0.0 injected for AMD ROCm
- Polls up to 30s for readiness after auto-start
- New env var: COMFYUI_DIR (default ~/ComfyUI)
- FastMCP lifespan= wired in; 34/34 tests still passing

Option B: Add comfyui.service systemd user service file
- Install: cp mcp/mcp-image-gen/comfyui.service ~/.config/systemd/user/
- Enable: systemctl --user enable --now comfyui
- Sets HSA_OVERRIDE_GFX_VERSION=11.0.0, WorkingDirectory=%h/ComfyUI
- Restart=on-failure, logs via journald

docs: Update mcp-image-gen-ComfyUI-Setup.md
- New Step 4: systemd service install + linger instructions
- Step 5: manual start (moved from old Step 4)
- Step 6/7 renumbered; COMFYUI_DIR env var documented
- Architecture diagram added; troubleshooting rows updated

mcp/mcp-image-gen/comfyui.service

@@ -0,0 +1,21 @@
+[Unit]
+Description=ComfyUI — Local AI Image Generation (AMD ROCm / FLUX.1-schnell)
+Documentation=https://github.com/comfyanonymous/ComfyUI
+After=network.target
+
+[Service]
+Type=simple
+WorkingDirectory=%h/ComfyUI
+ExecStart=%h/ComfyUI/.venv/bin/python main.py --listen --port 8188
+Restart=on-failure
+RestartSec=10
+
+# AMD RX 7900 XTX ROCm GFX override — required for correct GPU detection
+Environment=HSA_OVERRIDE_GFX_VERSION=11.0.0
+
+# Redirect output — follow with: journalctl --user -u comfyui -f
+StandardOutput=journal
+StandardError=journal
+
+[Install]
+WantedBy=default.target