dsh-web-search-doubao: Doubao Search Provider for DeepSeek Harness

`dsh-web-search-doubao` is a community plugin that provides the Volcengine Doubao search service for DeepSeek Harness (DSH). It integrates via the `ctx.web` slot, supports separate billing, and does not consume model tokens. The plugin is available in both `custom` and `global` variants, supports runtime configuration, and securely stores API keys. After installation, users can invoke the `web_search` tool to retrieve search results. As a non-Ark plugin, it is fully decoupled from model inference and aims to enrich the DSH ecosystem.