feat: adapt to DSH 0.1.2-alpha.5 and bump to v0.19.0-alpha.0

- pins: CI mount-gate CLI, dsh.plugin.json engines floor, and the
  @deepseek-ai/* peer floor / devDependencies baseline move to
  0.1.2-alpha.5 (dsh-client-locale trails at 0.1.2-alpha.3 — no such
  release upstream; ^0.1.2-alpha.3 already accepts the alpha.5 runtime)
- code: migrate the alpha.4 Session.events removal to snapshotEvents()
  (sidechat transcript live reads, fork inheritance, jobs.output replay,
  subagent activity) and drop the seedLength meta field the host removed
  from CreateAgentOptions.meta
- gates: pnpm peers check clean, typecheck + 1137 unit tests green,
  real-host mount smoke 14/14 on npm-published 0.1.2-alpha.5

dsh.plugin.json

@@ -1,10 +1,10 @@
 {
   "id": "dsh-external/dsh-better-sidebar",
-  "version": "0.18.1-alpha.0",
+  "version": "0.19.0-alpha.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.3"
+    "dsh": ">=0.1.2-alpha.5"
   },
   "contributes": {
     "tools": [],