chore(baseline): target DSH 0.1.6-alpha.2 and drop 0.1.5-rc support

The peer floor had to move: semver's prerelease rule needs a comparator on the
same [major,minor,patch] tuple, so `^0.1.5-rc.1` can never match any 0.1.6
prerelease. The floor becomes `^0.1.6-alpha.2` (which does admit 0.1.6 alpha/rc
prereleases and excludes 0.1.5), and `engines.dsh` follows. This is a deliberate
support-line break, matching how v0.19.0 dropped 0.1.2: 0.1.5-rc hosts stay on
0.19.1.

Three upstream changes actually reach this plugin and are adapted here:

- `IconSendOutline16` no longer exists in ui-primitives (only
  `IconSendOutline14`).
- `TerminalBlockLabels` gained a required `noExitCode` pill text.
- `ConnectionIndicator` dropped `reconnectLabel`: the outage label is now the
  one static label that already names the retry action, so the label copy moves
  there, matching the host's own '连接异常，点击立即重连'.

ui-primitives still declares no `dependencies` while its bundle bare-imports
more than it used to: `diff` and `simple-icons` were previously satisfied only
transitively and had to be hoisted into devDependencies, exactly like the
anser/shiki/katex group before them. `@deepseek-ai/dsh-code-runtime` is gone
upstream (the package no longer exists) and leaves the dev tree.

The e2e fallback npx spec is pinned rather than version-less: an unpinned
`npx @deepseek-ai/dsh` resolves npm's `latest`, which is still the 0.1.5 line
this plugin no longer supports.

dsh.plugin.json

@@ -1,10 +1,10 @@
 {
   "id": "dsh-external/dsh-better-sidebar",
-  "version": "0.19.1",
+  "version": "0.20.0",
   "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": ">=0.1.5-rc.1"
+    "dsh": ">=0.1.6-alpha.2"
   },
   "contributes": {
     "tools": [],