refactor: migrate built-in plugins to SDK-only interface

- Six-phase plan complete: webui/cli/healthcheck/pluginmgr/clawhubadapter
  now interact with the kernel exclusively via internal/sdk interfaces;
  all Configure() calls and package-level global injection removed
- buildSDK in internal/plugin/registry.go is the single assembly point
- Add internal/sdk/events.go exporting event types/constants
- Fix ProviderManager cooldown sharing: LuaAdaptedProvider.Name() now
  returns the source name instead of lua_<adapter>, so multiple sources
  sharing an adapter (single script load via shared VM AdapterCache) no
  longer share failure-cooldown state
- Verified: build/vet/tests green, deployed to homeagent.service with
  full plugin capability testing via local OpenAI-compatible mock

internal/sdk/knowledge_impl.go

@@ -26,4 +26,14 @@ func (k *knowledgeImpl) List() ([]string, error) {
 	return k.ks.List(), nil
 }
 
+func (k *knowledgeImpl) Stats() map[string]interface{} {
+	if k.ks == nil { return map[string]interface{}{} }
+	return k.ks.Stats()
+}
+
+func (k *knowledgeImpl) Remove(name string) error {
+	if k.ks == nil { return nil }
+	return k.ks.Remove(name)
+}
+
 var _ KnowledgeAPI = (*knowledgeImpl)(nil)