Debug: fix headless mode, plugin compat, psutil optional, module imports

- headless: TUI skip now works correctly
- example_plugin: keyword args compat (plugin_name=, config=, bridge=)
- sysmon: psutil made optional (graceful degrade)
- tui_service: SysMonWidget optional
- All 22 modules import clean, 23 tests pass, integration verified

services/sysmon_widget.py

@@ -1,6 +1,10 @@
 #!/usr/bin/env python3
 """SenSu TUI 系统监控组件 — CPU/内存/磁盘/插件状态"""
-import psutil, time, asyncio, logging
+try:
+    import psutil
+except ImportError:
+    psutil = None
+import time, asyncio, logging
 from textual.widgets import Static
 from textual.reactive import reactive