license: SDK 改用 MIT —— 让第三方插件不被 AGPL 传染

原 LICENSE 是 2026-09-12（93ab794）引入的 AGPL-3.0-only 全文，README 据此
声明「插件静态链接 SDK，故必须以相同许可发布，闭源只能另取商业授权」。
这堵死了闭源插件，与第三方插件生态的目标相反。

## 为什么 MIT 才自洽

SDK 会随插件一起静态链接（源码进入插件二进制）。用传染许可，插件作者就被
强制开源；用 MIT，插件作者可自由选择许可（闭源 / 商业 / 私有均可），无需
回馈、无需任何例外或商业授权。这是刻意的宽松，也是插件生态安全的基础。

MIT 在这里**不会与任何许可冲突**：SDK 完全自包含 —— go.mod 零外部依赖，
sdk/ 只 import Go 标准库（sync），不引用核心仓任何代码（已实测确认）。

## 改动

- LICENSE：AGPL-3.0-only 全文（661 行）→ MIT 正文（21 行，版权人 JianFeeeee）
- README.md / README_EN.md：许可章节重写，把「必须同许可」改为「可自选许可」，
  并写明自包含这一前提

内核仓仍为 AGPL-3.0-only（其 README 的对应论述与本提交同批更新）。

验证：go build ./sdk/... ./meta/... 通过。

README_EN.md

@@ -849,14 +849,18 @@ Or upload via the WebUI plugin management page, or manually place the `.hmap` in
 
 ## License
 
-The SDK is released under **AGPL-3.0-only** — see [LICENSE](LICENSE).
+The SDK is released under the **MIT license** — 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.
+**This permissiveness is deliberate**: the SDK is **statically linked** into your plugin
+(its source ends up in the plugin binary). Under a copyleft license such as AGPL that would
+force plugin authors to open-source their work; MIT exists precisely so that plugin authors
+can **pick their own license** — closed-source, commercial or private — with no obligation to
+contribute back and no need for any exception or commercial grant. The safety and vitality of
+the third-party plugin ecosystem rest on this.
+
+This is sound because the SDK is **fully self-contained**: `go.mod` has zero external
+dependencies and `sdk/` imports only the Go standard library (`sync`), never any code from the
+core repository — so the MIT grant conflicts with nothing.
 
 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