feat(dsh): adapt to DSH 0.1.5-alpha.1

Move the plugin onto the DSH 0.1.5-alpha.1 baseline (peer floor
^0.1.5-alpha.1, CI pinned to the alpha dist-tag) and fix the two host
contract changes that alpha line introduced.

Baseline:
- all @deepseek-ai/dsh-* peer/dev deps -> 0.1.5-alpha.1; optional peer
  @deepseek-ai/dsh-client-ui-sidebar-right (declared, unused until the
  native-surface phase); devDep @deepseek-ai/dsh-agent-loop (inbox fold)
- dsh.plugin.json engines >=0.1.5-alpha.1; version 0.19.0-alpha.0;
  SIDEBAR_SERVICE_VERSION follows

Live assistant stream (assistant/chunk is gone):
- 0.1.5 publishes in-flight model deltas as process-local
  agent/assistant-stream frames instead of durable assistant/chunk events
  and embeds the settled stream in assistant/message / assistant/attempt.
  src/assistant-live.ts folds the frames into a bounded per-session buffer;
  sidechat.events serves the current attempt as `live` rows (replaced every
  poll, never a delta) and the transcript renders them; the inherited
  in-progress snapshot reads message content / attempt stream / live frames.

Cold session reads (persistence.inspect is gone):
- 0.1.5 reads a stored session through an explicit read handle
  (open(id,'read') -> handle.read() -> close()). src/session-store.ts owns
  that protocol; every cold read (sidechat transcript, cold-resume preset,
  session.cwd, changes replay) goes through it.

Dead code and docs:
- drop the remote.session.openWorkspacePath interception: 0.1.5's chat opens
  files through ctx.sidebarRight.openResource instead, so the wrapper had no
  caller and would hijack the host's "open in app" gesture; the
  interceptOpenPath pref, its settings row and its locale keys go with it
- AGENTS.md §3 + external-plugin-guide.md describe the new baseline
- vitest setup bridges Node's localStorage accessor to jsdom's store (the
  pre-existing 26 failures on Node >= 22)
- desktop-layout e2e seeds through host RPC and types into the Lexical
  composer (0.1.5 replaced the textarea) instead of driving the directory
  picker

Verified: pnpm typecheck / lint / build / test (124 files, 1327 tests) /
check:consumer-types, plus pnpm test:mount against a real
@deepseek-ai/dsh@0.1.5-alpha.1 host (18/18).

src/client/locales.ts

@@ -249,8 +249,6 @@ export const zh = {
   settingsWidthTitle: '默认宽度占比',
   settingsWidthDesc: '新建会话时侧边卡片占窗口宽度的百分比 (20–60)',
   settingsWidthSuffix: '%',
-  settingsOpenPathTitle: '聊天区文件在侧边栏打开',
-  settingsOpenPathDesc: '在聊天里点击文件链接（工具行、产物列表、文件提及）时，在侧边栏编辑器中打开，不再调用系统默认应用',
   settingsOpenToolsTitle: '为模型注入侧边栏打开工具',
   settingsOpenToolsDesc: '开启后，模型可通过 sidebar_open 工具在侧边栏主动打开文件、文件夹和 HTTP(S) 网页（默认关闭）',
   settingsTitleBarTitle: '位置兼容模式',
@@ -699,8 +697,6 @@ export const en: Record<keyof typeof zh, string> = {
   settingsWidthTitle: 'Default width share',
   settingsWidthDesc: 'The side card\'s default share of the window width for new conversations (20–60)',
   settingsWidthSuffix: '%',
-  settingsOpenPathTitle: 'Open chat files in the sidebar',
-  settingsOpenPathDesc: 'Open file links in the chat (tool rows, produced files, mentions) in the sidebar editor instead of the system default app',
   settingsOpenToolsTitle: 'Inject the sidebar-open tool for the model',
   settingsOpenToolsDesc: 'When enabled, the model can actively open files, folders, and HTTP(S) pages in the sidebar through the sidebar_open tool (off by default)',
   settingsTitleBarTitle: 'Position compatibility mode',