feat(knowledge): 阶段二 向量 RAG 与召回注入

- embeddings.py：OpenAI 兼容 embedding 客户端，未配置时关键词兜底
- chunking.py + knowledge_embeddings 表：Markdown 分块与向量索引（Python 余弦）
- search.py：keyword/vector/hybrid 三种模式，向量不可用自动降级
- 笔记创建/更新自动重建向量索引；新增 /reindex 维护接口
- KnowledgeRagMiddleware：按对话级开关 knowledge_rag_enabled 注入 <knowledge-context>，接入 lead agent 中间件链（默认随 config.knowledge.rag_enabled）
- service.build_rag_context（wiki-query 风格分层召回）
- 笔记版本快照（knowledge_note_versions）+ GET /notes/{id}/versions
- 前端：聊天页「知识库」对话级 RAG 开关 + 类型字段
- 新增 entities/relations 表（阶段四预置）

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

offline-backend-20260512/backend/packages/harness/deerflow/agents/lead_agent/agent.py

@@ -299,6 +299,16 @@ def _build_middlewares(
     if not is_bootstrap and not is_writing_setup:
         middlewares.append(MemoryMiddleware(agent_name=agent_name, memory_config=resolved_app_config.memory, injection_enabled=memory_injection_enabled))
 
+    # KnowledgeRagMiddleware — inject knowledge-base recall (phase 2). Gated
+    # per-conversation by the ``knowledge_rag_enabled`` runtime flag, defaulting
+    # to the global ``knowledge.rag_enabled`` config. No-op when knowledge is
+    # disabled. Skipped in bootstrap/writing-setup like MemoryMiddleware.
+    knowledge_cfg = getattr(resolved_app_config, "knowledge", None)
+    if not is_bootstrap and not is_writing_setup and knowledge_cfg is not None and knowledge_cfg.enabled:
+        from deerflow.agents.middlewares.knowledge_rag_middleware import KnowledgeRagMiddleware
+
+        middlewares.append(KnowledgeRagMiddleware(limit=knowledge_cfg.rag_limit, default_enabled=knowledge_cfg.rag_enabled))
+
     # Add ViewImageMiddleware only for the NATIVE-vision path (main model
     # ingests raw image data). In delegated mode the view_image tool already
     # returns a text description and stores nothing in `viewed_images`, so the