docs(skills): add smart-search skill

[tiaot33]

Description

新增一个 smart-search skill，用于指导 agent 在使用 opencli 时进行搜索路由。

这个 skill 主要解决以下问题：

• 当用户明确指定网站时，直接使用对应网站
• 当用户没有指定网站时，优先从 grok、doubao、gemini 中选择一个 AI 源
• 当 AI 返回内容不足时，再补充专用站点
• 先通过 opencli list -f yaml 做预检，确认当前环境里可用的站点
• 通过 opencli <site> -h 和 opencli <site> <command> -h 读取实时帮助，避免 skill 文档和命令实
  际行为漂移
• 为 AI 搜索增加搜索词建议，减少模糊查询

本次新增内容包括：

• skills/smart-search/SKILL.md
• skills/smart-search/references/sources-ai.md
• skills/smart-search/references/sources-tech.md
• skills/smart-search/references/sources-social.md
• skills/smart-search/references/sources-media.md
• skills/smart-search/references/sources-info.md
• skills/smart-search/references/sources-shopping.md
• skills/smart-search/references/sources-travel.md
• skills/smart-search/references/sources-other.md

Related issue:

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

新增文件：

• skills/smart-search/SKILL.md
• skills/smart-search/references/sources-ai.md
• skills/smart-search/references/sources-tech.md
• skills/smart-search/references/sources-social.md
• skills/smart-search/references/sources-media.md
• skills/smart-search/references/sources-info.md
• skills/smart-search/references/sources-shopping.md
• skills/smart-search/references/sources-travel.md
• skills/smart-search/references/sources-other.md