feat: add AI endpoint manager plugin with configuration and routing capabilities

- Introduced `ai_endpoint_mgr` plugin to manage multiple AI provider endpoints.
- Added configuration reference documentation for `config.toml`.
- Implemented endpoint loading, active endpoint switching, and model mutation.
- Included error handling for missing endpoints and configuration failures.
- Developed unit tests covering various scenarios including error paths and concurrency.

docs/tutorial/quick-start.md

@@ -96,6 +96,8 @@ api.model = "gpt-4o"
 > **关键**: 修改 `ai.provider` 字段即可在不同后端间切换, 无需改动代码。
 >
 > API Key 可从 [OpenAI-compatible 开放平台](https://platform.openai.com/) 或 [Anthropic Console](https://console.anthropic.com/) 获取。
+>
+> **多 Endpoint 配置** (同时使用多个 AI 后端并在运行时通过 `/endpoint` 切换): 参见 [配置参考](../reference/config.md) 和项目根目录 `config.example.toml`。
 
 ---
 
@@ -172,5 +174,6 @@ dstalk v0.1.0  |  dstalk AI  |  /help 查看帮助  |  /quit 退出
 ## 下一步
 
 - 查看 [CLI 命令速查表](../reference/commands.md) 了解全部命令
+- 查看 [配置参考](../reference/config.md) 了解 config.toml 所有字段
 - 输入 `/help` 在 dstalk 内查看命令列表
 - 输入 `/status` 查看当前运行状态