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/background/thinker.go

@@ -135,7 +135,7 @@ func DefaultAutonomousToolPolicy() *AutonomousToolPolicy {
 			"iot_query", "iot_control", "memory_search", "web_search",
 			"calculator", "datetime", "web_fetch",
 			"host_exec", "host_file", "host_system",
-			"vision_analyze",
+			"vision_analyze", "knowledge_search", "knowledge_ingest",
 		},
 		MaxToolCallsPerRound: 5,
 		MaxHighRiskPerHour:   10,