feat(元宝): add browser adapter and docs

[tiaot33]

Description

新增 yuanbao 浏览器适配器，支持以下命令：

• opencli yuanbao new
• opencli yuanbao ask <prompt>

本次改动同时补齐了适配器文档、侧边栏入口、README 命令可发现性，以及登录缺失场景下的 E2E 失败兜底测试。

实现要点：

• 新增 yuanbao new，优先点击页面中的新建会话入口，失败时回退到首页重载
• 新增 yuanbao ask <prompt>，支持 --timeout、--search、--think
• 发送前会对齐 联网搜索 和 深度思考 的开关状态
• 通过转录增量和稳定轮询提取最终回复，尽量规避回显、占位文案和中间态干扰
• 补充 turndown 转换规则，支持将表格类回复转为 Markdown
• 将预期失败标准化为 CliError 子类：
  • 登录墙改为 AuthRequiredError
  • 发送失败改为 CommandExecutionError
  • 无响应超时改为 TimeoutError

Related issue:

N/A

Type of Change

• 🐛 Bug fix
• ✨ New feature
• 🌐 New site adapter
• 📝 Documentation
• ♻️ Refactor
• 🔧 CI / build / tooling

Checklist

• I ran the checks relevant to this PR
• I updated tests or docs if needed
• I included output or screenshots when useful

Documentation (if adding/modifying an adapter)

• Added doc page under docs/adapters/ (if new adapter)
• Updated docs/adapters/index.md table (if new adapter)
• Updated sidebar in docs/.vitepress/config.mts (if new adapter)
• Updated README.md / README.zh-CN.md when command discoverability changed
• Used positional args for the command's primary subject unless a named flag is clearly better
• Normalized expected adapter failures to CliError subclasses instead of raw Error

Screenshots / Output