feat: Phase 6.6 知识库 RAG 增强 — 文档索引 + 语义检索 + KnowledgeProvider

- rag.Embedder: LLM API 文本向量化 (OpenAI-compatible)
- rag.KnowledgeStore: 文档分块 + 重叠窗口 + 余弦相似度搜索
- rag.Retriever: 高级知识检索 + 格式化摘要
- KnowledgeProvider: 子会话提供者，整合入编排管线
- knowledge_search / knowledge_ingest 工具
- EnrichmentData 管线全线支持 KnowledgeInfo

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

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

@@ -0,0 +1,61 @@
+package rag
+
+import (
+	"context"
+	"fmt"
+	"strings"
+)
+
+// Retriever provides a high-level knowledge retrieval interface.
+type Retriever struct {
+	store *KnowledgeStore
+}
+
+// NewRetriever creates a new knowledge retriever.
+func NewRetriever(store *KnowledgeStore) *Retriever {
+	return &Retriever{store: store}
+}
+
+// Retrieve searches the knowledge base and returns formatted results.
+func (r *Retriever) Retrieve(ctx context.Context, query string, topK int) (*RetrievalResult, error) {
+	results, err := r.store.Search(ctx, query, topK)
+	if err != nil {
+		return nil, fmt.Errorf("knowledge search: %w", err)
+	}
+
+	ret := &RetrievalResult{
+		Query:   query,
+		Results: results,
+		Summary: r.buildSummary(results),
+	}
+	return ret, nil
+}
+
+// RetrievalResult holds knowledge retrieval output.
+type RetrievalResult struct {
+	Query   string         `json:"query"`
+	Results []SearchResult `json:"results"`
+	Summary string         `json:"summary"`
+}
+
+func (r *Retriever) buildSummary(results []SearchResult) string {
+	if len(results) == 0 {
+		return "知识库中未找到相关信息。"
+	}
+	var sb strings.Builder
+	sb.WriteString(fmt.Sprintf("从知识库中找到 %d 条相关信息:\n\n", len(results)))
+	for i, result := range results {
+		sb.WriteString(fmt.Sprintf("--- 来源: %s (段落 %d, 相关度 %.0f%%) ---\n",
+			result.Chunk.DocTitle, result.Chunk.Index+1, result.Score*100))
+		sb.WriteString(result.Chunk.Content)
+		if i < len(results)-1 {
+			sb.WriteString("\n\n")
+		}
+	}
+	return sb.String()
+}
+
+// Stats returns knowledge base statistics.
+func (r *Retriever) Stats() map[string]interface{} {
+	return r.store.Stats()
+}