Vector Search: sqlite-vec + BGE-small-zh 集成

[DankerMu]

Part of #8

Dependencies: #10 (MVP-β 完成)

Description

集成 sqlite-vec 扩展实现零额外部署的向量检索，按段落切分 (~200 字/段)，使用本地 BGE-small-zh 模型嵌入，结果与 KG evidence 去重后注入 Context Pack。

OpenSpec Spec: `vector-search`

Tasks

• 4.1 Integrate sqlite-vec extension
• 4.2 Implement paragraph chunking (~200 chars/chunk)
• 4.3 Integrate BGE-small-zh local embedding model
• 4.4 Implement vector search API with deduplication against KG evidence
• 4.5 Add vector results to Context Pack (5~8% budget, supplementary layer)
• 4.6 Write tests for embedding, search, deduplication

Acceptance Criteria

• sqlite-vec 扩展正常加载
• 段落嵌入和检索延迟 <200ms
• 向量结果与 KG evidence 按 paragraph_idx 去重
• 向量层独立预算 5~8%