feat(dispatcher): Eino 采纳 Phase A —— llm.Pool 内部换成 Eino ChatModel 组件

把手写的 OpenAI HTTP/SSE 客户端换成 eino-ext/openai 的 ChatModel 组件：
- SetConfig 用激活配置构建 ChatModel（热更新=重建实例，读写锁保护）
- ChatStream → cm.Stream（StreamReader 逐 chunk）；Chat → cm.Generate
- 对外签名零变更（Chat/ChatStream/StreamText/Ready/ModelName），
  graph.go / report.go / memory_extract.go 不动
- Ready() 语义升级为「ChatModel 构建成功」，更准
- StreamText 降级桩保留

验收：make test-go 全模块绿；真实链路提交任务经新组件流式出答复
（28 字 + eval 1.00，Stream/Generate 两条路径都过）。

为 Phase B（BindTools 函数调用 + ADK）铺好底层组件。

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

go.work.sum

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