docs(license): SDK 采用 AGPL-3.0-only，并补许可章节

本仓此前**没有任何许可文件**，README 里也没有许可声明。现补：

- `LICENSE`：GNU Affero 通用公共许可证第 3 版官方全文（gnu.org 正本，
  661 行 / 34523 字节 / sha256 0d96a4ff68ad6d4b6f1f30f713b18d5184912ba8dd389f86aa7710db079abcb0）
- README.md / README_EN.md：新增许可章节

选 AGPL-3.0-only 的理由：GPL 家族里传染性最强的一档，且不允许选后续版本。
对插件开发者的实际含义已写进 README：SDK 随插件静态链接（源码进入插件二进制），
插件因此是本 SDK 的衍生作品，必须以相同许可发布；AGPL §13 也覆盖网络交互，
通过 HTTP/WebSocket 提供服务的插件同样要向使用者提供源码。

第三方（Go 依赖 go-sqlite3 / gojieba / bubbletea 等，MIT / BSD-3 / Apache-2.0）
保持各自许可；平台侧模型与运行时（Chinese-CLIP Apache-2.0、ONNX Runtime MIT）
不属于本 SDK。

README_EN.md

@@ -747,3 +747,19 @@ curl -X POST http://127.0.0.1:9876/plugins \
 ```
 
 Or upload via the WebUI plugin management page, or manually place the `.hmap` in the plugin directory and restart the platform.
+
+## License
+
+The SDK is released under **AGPL-3.0-only** — see [LICENSE](LICENSE).
+
+**This is a substantive constraint for plugin developers**: the SDK is **statically linked** into
+your plugin (its source ends up in the plugin binary), so the plugin is a derivative work of
+this SDK and **must be released under the same license**. Because AGPL §13 covers network
+interaction, a plugin that serves users over HTTP/WebSocket must also offer them the source.
+If you need a closed-source plugin, the only compliant route is a separate exception/commercial
+license from this project — none is offered today.
+
+Third-party components (Go dependencies: go-sqlite3, gojieba, bubbletea, … — MIT / BSD-3 /
+Apache-2.0) keep their own licenses. The platform-side model and inference runtime
+(Chinese-CLIP Apache-2.0, ONNX Runtime MIT) are not part of this SDK; their full license texts
+ship with the release packages under `/usr/share/doc/homeagent/licenses/`.