feat: Zvec / zvec-grep (zg) backend for semantic codebase_search (#4066)

[Hmbown]

Context & Proposal

Alibaba open-sourced zvec (an in-process vector DB built on Proxima, Apache-2.0) and zvec-grep (zg: unified ripgrep + BM25 + vector search CLI with local-first posture & MCP surface).

This fits issue #4066 (feat: semantic codebase_search tool), currently config/icon-only:

• Local-first hybrid search: zg unifies exact/regex, BM25, and local vector search into ranked, source-linked results.
• Provider neutrality & DashScope integration: Combines with local embedding models or Qwen embeddings via DashScope API key.
• Token efficiency: Reduces tool calls by delivering line-level cited evidence directly for agent consumption.

Spike Plan

1. Evaluate zg CLI & MCP adapter in local environment.
2. Prototype codebase_search tool wrapper invoking zg query / MCP server.
3. Compare search accuracy, latency, and context budget efficiency vs BM25 tool_search.

Ref: #4066