feat(oaiapp): 让项目能支持 OpenAI-compatible /v1/chat/completions

[RobinRush]

Summary

• 新增 frontends/oaiapp.py：把整个智能体（带工具循环、分层记忆等完整行为）作为一个 OpenAI 风格"模型"暴露出去，任何兼容 OpenAI 接口的客户端（NextChat / Open WebUI / LobeChat / OpenAI SDK / curl 等）即可接入。
• 支持流式（SSE）与非流式两种模式；零新增依赖（复用已有的 bottle + 标准库 wsgiref 多线程服务器）。
• launch.pyw 增加 --api 启动开关；mykey_template.py 增加配置示例段；README.md 中英双语补充文档。

Endpoints

• POST /v1/chat/completions — Chat Completions，支持 stream: true/false
• GET /v1/models — 列出当前配置的全部 LLM 后端

Configuration (mykey.py)

oai_api_token   = 'sk-local-your-secret-token'   # 必填，Bearer token
# oai_api_host  = '127.0.0.1'                    # 可选；'0.0.0.0' 暴露到局域网
# oai_api_port  = 18000                          # 可选
# oai_api_timeout = 600                          # 可选，单请求超时秒数

[lsdefine]

Thanks for your contribution! This PR modifies 4 files with +431 lines of changes. To maintain our review quality and adhere to our modularity principles, we prefer smaller, focused PRs. Please consider breaking this into more targeted pieces and resubmitting.