chore(release): cut v0.21.1 as the stable adapter for DSH 0.1.7-rc.1

The rc.1 adapter first went out as 0.21.0-rc.1, but DSH 0.1.7-rc.1 is
upstream's release batch, so the plugin that targets it must not sit on a
prerelease channel. Re-cut it as v0.21.1: the version is now free of `-`,
so release.yml publishes it under npm's `latest` instead of `alpha`.

Version literals only — package.json, dsh.plugin.json and
SIDEBAR_SERVICE_VERSION (tests/service.spec.ts still pins the last two
together). No code change: the peer floor stays ^0.1.7-rc.1 and the
support line stays "DSH 0.1.7-rc.1 and later".

Docs follow the channel move: README/README_EN/guide drop the "alpha
channel" framing, the install command becomes @latest, and every older
DSH line now gets an explicitly pinned command in the version table
(@0.19.1 and so on) because `latest` no longer covers them. The rc.1
design record keeps its history and gains a §8 for the re-cut.

dsh.plugin.json

@@ -1,6 +1,6 @@
 {
   "id": "dsh-external/dsh-better-sidebar",
-  "version": "0.21.0-rc.1",
+  "version": "0.21.1",
   "main": "./lib/index.js",
   "description": "更好用的右侧边栏——文件树、编辑器、文件变动、任务和侧边对话都收在这一栏里，每个会话互不干扰。暴露服务供其他插件注册侧边栏页面与文件预览器",
   "engines": {