dsh-subscription-search: Search Chain Plugin for DeepSeek Harness

`dsh-subscription-search` is a search-only plugin for DeepSeek Harness (DSH), designed to address the limitations of relying on a single search source. It provides multi-source search-chain services, with automatic fallback and invocation for search sources such as ChatGPT, Grok, Ollama, Exa, and DeepSeek. The plugin includes a built-in default search chain, supports dynamic adapter registration, and offers comprehensive chain-management capabilities. It does not handle OAuth or credential storage, focusing solely on search strategy and state management. Installation requires specific DSH versions (0.1.2-alpha.3/0.1.2-rc.1/0.1.5-rc.1) and supports status checking, uninstallation, and installation via the CLI.