license: 明确内核 AGPL / SDK MIT 的边界（插件不受 AGPL 传染）

SDK 仓改为 MIT（同批提交 e97cafc）后，核心仓几处「插件静态链接 SDK 故须同许可」
的论述不再成立，一并更正。

## 边界（现在写清楚了）

- **AGPL-3.0-only 覆盖内核与随包内置插件**：homed、internal/、internal/plugins/
  下 18 个内置插件。§13 网络条款照旧适用。
- **MIT 覆盖公开 SDK**：sdk/ 零外部依赖、只依赖 Go 标准库，不引用内核任何代码。
- 因此**外部插件不是本项目的衍生作品**，作者可自行选择许可（含闭源、商业、
  私有），既不必同许可、也不受 §13 约束。

## 改动

- README.md / README_EN.md「许可」章节：改写静态链接那段，写明上述边界与理由
  （决定许可的是被链接的 SDK 代码，而它是 MIT；子进程隔离不再是关键论点）
- README.md / README_EN.md changelog：**按本仓惯例保留历史原文**，在既有更正
  注记区追加一条「许可口径已变更（2026-09-24）」，而不是去改写 v1.2.0 的原文
  —— 避免伪造当时的语境
- site/index.html footer：状态栏拆成「内核许可：AGPL-3.0-only」+「插件 SDK：MIT」
  两行；声明区分「AGPL（内核与其内置插件）」与「SDK 以 MIT 发布，插件不受传染」
- .gitignore：example/ 注释里的计数 20 → 21（实际 10 个示例 ×2 + qq/plugin_test.go；
  规则本身与「有意保留」的说明不变）

验证：go build ./internal/meta/ ./internal/agent/core/ 通过；官网 1440px 无横向溢出、
无控制台错误，footer 许可两行正确渲染（/tmp/lic-footer.png）。

README_EN.md

@@ -264,6 +264,12 @@ External plugin development: see [homeagent-sdk](https://gitcode.com/JianFeeeee/
 > The historical entries below are kept verbatim to show the evolution; two mechanisms in them
 > were **removed in v1.2.0**: text-description-based media indexing, and reference-counted media GC.
 >
+> **One licensing statement has also changed** (2026-09-24): "statically linked plugins must match"
+> in the v1.2.0 entry below **no longer holds**. The SDK is now released under **MIT** (a permissive
+> license that does not inherit the kernel's AGPL), so external plugins are **not** derivative works
+> of this project: authors choose their own license (closed-source, commercial or private included)
+> and are not bound by §13. The original text is kept to show the position at the time.
+>
 > **Two performance claims also need correcting** (measured 2026-09-20):
 > "crash-to-recovery under 1s" does not hold — restarts are **linearly backed off**, i.e.
 > 1s / 2s / 3s (`procRestartBackoff=1s × nth crash`). Even the *first* restart waits 1s.
@@ -353,9 +359,19 @@ source when you distribute the software, **you must also offer the source to use
 with it over a network** (§13, Remote Network Interaction). Anyone running a modified HomeAgent
 as a network service therefore has to make the modified source available to that service's users.
 
-Plugins are **statically linked** against this project through the public SDK (the SDK source
-ends up inside the plugin binary), so plugins are derivative works and must be released under
-the same license. Process isolation does not change this — what is linked is the SDK code itself.
+Plugins are **statically linked** against this project through the public
+[homeagent-sdk](https://gitcode.com/JianFeeeee/homeagent-sdk) (the SDK source ends up inside the
+plugin binary), but that repository is released under **MIT** — a permissive license, so code
+received under it does **not** inherit this project's AGPL. External plugins are therefore **not
+derivative works of this project**: authors pick their own license (closed-source, commercial or
+private included), with no same-license obligation and no §13 network clause. The safety and
+vitality of the third-party plugin ecosystem rest on this.
+
+The boundary is clean: **AGPL covers the kernel and the bundled plugins** (`homed`, `internal/`,
+the 18 built-in plugins under `internal/plugins/`); **MIT covers the public SDK** (`sdk/`, whose
+`go.mod` has zero external dependencies and imports only the Go standard library — it never
+references any kernel code). Process isolation is beside the point here — what decides the
+license is the linked SDK code itself, and that code is MIT.
 
 ### Third-party components shipped with the packages