feat: 设置页「添加插件」推荐目录 + 复制安装命令；移除内置 Office 预览

- 设置页侧边栏内容/文件预览两个网格末尾各增虚线卡片，弹出对应 kind 的插件弹窗
- 弹窗：GitHub topic 按钮（新标签页）+ 推荐插件目录（plugins-tabs.ts / plugins-viewers.ts）
- 每条目「跳转」（window.open 新标签页直达仓库）与「复制」（writeClipboard + 已复制反馈）
- 不做终端预填流：不关闭弹窗、不打开终端、无失败路径（install/installNotice/installError* 文案删除）
- 内置 Office 预览（docx/xlsx/pptx）整体移除，chunk 白名单 5→2（terminal/editor）
- 文案「内置插件」→「推荐插件」；AGENTS.md/README/设计文档同步

src/client/plugins-tabs.ts

@@ -0,0 +1,15 @@
+/**
+ * The built-in catalog of TAB-registration plugins (sidebar pages),
+ * shown in the "add tab plugin" modal (Side card settings → 侧边栏内容 grid
+ * → the dashed card). Adding an entry: append one object here (unique
+ * `id` = npm package name, `url` = GitHub repo, `description` =
+ * i18n-friendly, `install` = the full shell command pre-filled into the
+ * install terminal — it starts with `cd ~/.dsh` so the install runs with
+ * the DSH home as the working directory). Data integrity is guarded by
+ * `tests/plugin-list.spec.ts`.
+ */
+import type { PluginEntry } from './plugins-shared.ts'
+
+/** Tab-registration plugins (empty until a real one exists — the modal
+ *  renders the empty state and points at the GitHub topic). */
+export const builtinTabPlugins: readonly PluginEntry[] = []