fix: correct OpenClaw plugin architecture - notifications as sole registration path

- Remove SKILL tool registration with nil handler (was causing nil
  pointer in StageHost.ExecuteTool)
- Rewrite loadOCPlugin: notifications are the only registration path.
  Synchronously drain all buffered register notifications after
  waitReady, then start persistent notifyLoop for future registrations.
  ListTools is verification-only, no longer registers tools.
- notifyLoop: persistent via for n := range sp.NotifyChan(), lifecycle
  bound to sidecar process
- StageHost.ExecuteTool: add nil-handler check (defense-in-depth)
- Remove unused OCNamedParam, DrainNotify from sidecar.go

internal/plugins/openclaw/sidecar.go

@@ -50,14 +50,6 @@ type OCNotification struct {
 	Params  json.RawMessage `json:"params"`
 }
 
-// OCNamedParam 通知参数中至少包含 name 的结构
-type OCNamedParam struct {
-	Type string `json:"type"`
-	Data *struct {
-		Name string `json:"name,omitempty"`
-	} `json:"data"`
-}
-
 type sidecarProcess struct {
 	name    string
 	dir     string
@@ -313,17 +305,6 @@ func (s *sidecarProcess) CallTool(name string, args map[string]interface{}) (str
 	return sb, nil
 }
 
-// DrainNotify 排空通知通道
-func (s *sidecarProcess) DrainNotify() {
-	for {
-		select {
-		case <-s.notifyCh:
-		default:
-			return
-		}
-	}
-}
-
 func (s *sidecarProcess) Close() {
 	s.mu.Lock()
 	if s.closed || s.stopped {