feat: pinned terminal i18n + docs + v0.17.0 bump

M5 of pinned terminals (v0.17.0):
- README/README_EN: feature list adds pinned terminals
- AGENTS.md §3.4: terminal row documents pin capability
- Design doc: implementation deviations (M3 disconnect signal convergence,
  M4 railRevision state for reduceFor non-notify)
- package.json + dsh.plugin.json: 0.16.1 → 0.17.0
- i18n: 12 new keys synced across all 19 language dictionaries (done in M2)
- All client-side tests green (149/149); typecheck + build pass

dsh.plugin.json

@@ -1,6 +1,6 @@
 {
   "id": "dsh-external/dsh-better-sidebar",
-  "version": "0.16.1",
+  "version": "0.17.0",
   "main": "./lib/index.js",
   "description": "VSCode 风格右侧侧边栏：文件资源管理器 / 编辑器 / 终端 / Git / 浏览器，按会话隔离。暴露服务供其他插件注册侧边栏页面和文件预览器",
   "engines": {