fix: RAG IngestDirectory 跳过 persona.md 和 _index.md

避免人格文档在RAG检索中重复出现，persona.md已通过系统提示注入。

Co-Authored-By: Claude <noreply@anthropic.com>

backend/ai-core/internal/rag/knowledge_store.go

@@ -64,6 +64,12 @@ func (ks *KnowledgeStore) IngestDirectory(ctx context.Context) (int, error) {
 		if !isSupportedFile(ext) {
 			return nil
 		}
+		// Skip files that are injected elsewhere (persona.md → system prompt,
+		// _index.md → metadata only, not story content).
+		base := filepath.Base(path)
+		if base == "persona.md" || base == "_index.md" {
+			return nil
+		}
 		n, err := ks.IngestFile(ctx, path)
 		if err != nil {
 			return fmt.Errorf("ingest %s: %w", path, err)