plugin disable system: kernel→SDK PluginManager + WebUI/CLI

- New disabled_plugins table (name, disabled_at, disabled_by)
- SDK.PluginManager interface: DisablePlugin/EnablePlugin/ListDisabledPlugins
- Registry implements PluginManager, wired into PluginSDK
- WebUI: POST /api/v1/plugins/<name>/disable|enable + plugins page with toggle
- Disabling webui shows confirmation dialog
- CLI: /plugin disable <name> / /plugin enable <name>
- pluginmgr removePlugin sync-cleanup from disabled_plugins table

internal/plugins/pluginmgr/plugin.go

@@ -455,6 +455,11 @@ func (p *Plugin) removePlugin(name string) (interface{}, error) {
 		return map[string]interface{}{"error": err.Error()}, nil
 	}
 
+	// 同步清理禁用表
+	if Reg != nil {
+		Reg.EnablePlugin(name)
+	}
+
 	return map[string]interface{}{
 		"status": "removed",
 		"name":   name,