feat!: move to the alpha track — DSH 0.1.2-alpha.x only (v0.18.0-alpha.0)

DSH itself is still on its 0.1.2-alpha line (npm `alpha` dist-tag), so the
plugin mirrors it: v0.18.0-alpha.0 publishes under the npm `alpha`
dist-tag and supports DSH 0.1.2-alpha.x only. Stable-DSH (0.1.0-rc.8 ~
0.1.1-rc.2) users stay on v0.17.1 (npm `latest`). Verified end-to-end
against a real 0.1.2-alpha.2 host: full vitest suite, typecheck,
build/pack, consumer type surface, and the headless mount smoke 14/14
(the prerelease version also passes `dsh plugin add file:`).

- peer floor raised `^0.1.0-rc.8` -> `^0.1.2-alpha.2` (the mount install
  no longer warns: the old floor could not semver-match a prerelease
  host); dsh.plugin.json engines.dsh `>=0.1.2-alpha.2`.
- Dropped the pre-alpha compatibility layer:
  - e2e host RPC collapsed from the dot/slash dual dialect to slash-only
    (rpcAttempt; the 404-probe negotiation and its cache are gone), and
    parseLaunchUrl now requires the ?token= launch URL;
  - MarkdownText labels collapsed to the nested single shape (the cast
    became unnecessary against the alpha.2 declarations);
  - the defunct `@deepseek-ai/dsh-client-runtime` row is gone from the
    chunk externals allowlist (chunk-loader / tsdown / two specs) and
    from dsh.client.inject.
- market-manifest guard now allows an exact prerelease version (alpha
  cuts are not the market's verified `latest` target); release.yml picks
  the npm dist-tag from the version (prerelease -> `alpha`, else
  `latest`) so an alpha can never hijack `latest`.
- Docs retargeted: AGENTS.md §2/§3, README/README_EN badges + v0.18.0
  -alpha.0 section + install note (`dsh-better-sidebar@alpha`), guide.

docs/external-plugin-guide.md

@@ -2,7 +2,7 @@
 
 > 面向 **消费插件开发者**：如何让你的插件向 better-sidebar 注册新的侧边栏页面（tab）和文件类型预览器。
 >
-> 适用版本：**v0.4.0+**（`ctx.betterSidebar` 服务）；声明式设置 **v0.4.1+**；text/number 设置行 **v0.11.0+**；badge/生命周期/定向打开/插件设置/版本探测 **v0.12.0+**；select 设置行（`settingSelect`）与外链认领（`urlTarget`）**v0.13.0+**；统一 `@deepseek-ai/cordis` 类型基底 **v0.15.2+**；自由窗口（`floatWindows`）**v0.16.0+**；终端固定（pin）**v0.17.0+**。当前版本 v0.17.2。
+> 适用版本：**v0.4.0+**（`ctx.betterSidebar` 服务）；声明式设置 **v0.4.1+**；text/number 设置行 **v0.11.0+**；badge/生命周期/定向打开/插件设置/版本探测 **v0.12.0+**；select 设置行（`settingSelect`）与外链认领（`urlTarget`）**v0.13.0+**；统一 `@deepseek-ai/cordis` 类型基底 **v0.15.2+**；自由窗口（`floatWindows`）**v0.16.0+**；终端固定（pin）**v0.17.0+**。当前版本 **v0.18.0-alpha.0**（alpha 通道，仅支持 DSH 0.1.2-alpha.x；stable 线为 v0.17.1）。
 > 权威代码：`src/client/service.ts`（服务实现）、`src/client/builtins/`（内置 7 tab + 6 viewer 参考实现）、`lib/types/client/service.d.ts`（类型声明）。
 > 仓库开发规则（硬约束 / CI / 发版）见 [AGENTS.md](../AGENTS.md)。