feat: adapt to DSH 0.1.2-alpha.2 (npm-published) and bump to v0.17.2

DSH 0.1.2-alpha.2 is on npm under the `alpha` dist-tag, so the CI mount
pin and the @deepseek-ai/* devDependencies baseline move up from
0.1.1-rc.1/rc.2 (the dead dsh-client-runtime devDep goes away with
them). Verified end-to-end against a real 0.1.2-alpha.2 host: full
vitest suite (1106), typecheck, build/pack, consumer type surface, and
the headless mount smoke 14/14 — the token URL, slash RPC and
MarkdownText labels contracts are unchanged from alpha.1.

- dsh-settings dropped the runtime `settingsNamespace` export:
  namespaces are validated at compile time (SettingsNamespaceInput), so
  the host passes the 'dsh-better-sidebar' constant directly and the
  fake settings service in smoke.spec casts to the branded type.
- dsh-subagent bumped SUBAGENT_DESCRIPTOR_VERSION 2 -> 3: the version is
  stamped by the host package; the sidechat seed assertion now follows
  the constant instead of pinning a literal.
- Restored SessionEvent.ignorable and the RemoteError wrapping were
  verified to have no impact on this plugin.
- e2e scratch profiles (e2e-mount.sh / e2e-aggregate-mount.sh) add
  '@deepseek-ai/*' to minimumReleaseAgeExclude (mirroring the repo
  root): alpha releases are often <24h old when the lane runs, which
  pnpm 11's default minimumReleaseAge would refuse to install.
- vitest excludes .worktrees/** — local task worktrees carried stale
  code against the shared node_modules and broke on the devDeps bump.
- Version lockstep 0.17.2 (package.json, dsh.plugin.json,
  SIDEBAR_SERVICE_VERSION, README/AGENTS/guide docs).

dsh.plugin.json

@@ -1,6 +1,6 @@
 {
   "id": "dsh-external/dsh-better-sidebar",
-  "version": "0.17.1",
+  "version": "0.17.2",
   "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": {