dsh-search-index: DeepSeek Harness Sidebar Session Search Enhancement

The `dsh-search-index` plugin adds an independent search entry to the DeepSeek Harness (DSH) sidebar, helping to address the difficulty of searching through long conversation histories. It maintains a separate SQLite database, supports switching between title and content search modes, and aggregates search results by conversation, with filtering and sorting by user, reply, or tool type. It also supports snapshot import and export, automatic filtering of archived conversations, and safe index rebuilding, ensuring that it does not interfere with the official index. The plugin is easy to install and can be invoked with the `Ctrl+K` shortcut, providing an efficient experience for retrieving historical records.