Independently installable DeepSeek Harness plugins

Key features

• dsh-web-search — multi-provider web search for ctx.web: Baidu, Doubao, Tavily, Exa, Serper.
• Each directory under packages/ is its own dsh.bundle.
• web_fetch can use Tavily Extract, Exa Contents, or DSH built-in HTTP independently of the search backend.
• The repository root is not a plugin.
• Compatible with DeepSeek Harness 0.1.0-rc.7.