feat: adapt to DSH 0.1.2-rc.1 and bump to v0.18.0 (stable)

Graduate from the alpha track to the stable channel:

- Baseline: CI mount pin, dsh.plugin.json engines floor, and every
  @deepseek-ai/* peer floor / devDependencies pin move to 0.1.2-rc.1
  (published on the `next` dist-tag). alpha.5 -> rc.1 is a pure version
  bump in the harness (all package.json "version" lines, zero source
  delta), so no code adaptation; every host contract was re-verified
  unchanged and the real-host mount smoke passes.
- dsh-client-locale resumed publishing; its trailing-pin exception is
  gone — peers and devDeps align at 0.1.2-rc.1 (pnpm peers check clean).
- Version: fold the unpublished v0.18.1-alpha.0 / v0.19.0-alpha.0 bumps
  into the v0.18.0 stable release; SIDEBAR_SERVICE_VERSION follows
  package.json (asserted by tests/service.spec.ts).
- Docs: README/README_EN badges + channel notes + a full "since v0.17.1"
  changelog entry (the two folded alpha numbers are annotated), AGENTS.md
  §2/§3 baseline wording, external-plugin-guide version line, and the
  e2e-host-protocol header's re-verified list gains 0.1.2-rc.1.

dsh.plugin.json

@@ -1,10 +1,10 @@
 {
   "id": "dsh-external/dsh-better-sidebar",
-  "version": "0.19.0-alpha.0",
+  "version": "0.18.0",
   "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": {
-    "dsh": ">=0.1.2-alpha.5"
+    "dsh": ">=0.1.2-rc.1"
   },
   "contributes": {
     "tools": [],