dsh-experts: DeepSeek Harness Expert Directory Plugin

The "expert" concept in DSH (DeepSeek Harness) is introduced through the out-of-tree plugin `dsh-experts`. This plugin integrates a community-maintained "Expert Marketplace" catalog into the Web UI, providing browsing on the settings page, quick access in the chat input bar, an HTTP API, and Agent tools. Users can search for and add experts or expert groups from the catalog and invoke multi-role collaboration in conversations using an `@expert <id>` code block. The plugin depends on DSH 0.1.0-rc.6, caches data locally, does not read or write personal files, and treats all remote content as display- and validation-only, ensuring security.