dsh-session-explorer: Message-Level Search Plugin for DeepSeek Harness Sessions

`dsh-session-explorer` is a plugin designed specifically for the DSH web interface to address the difficulty of retrieving messages in long conversations. It supports full-text search across user messages, assistant replies, system-injected content, and tool invocation information, and can deduplicate results across sessions. The plugin provides a session overview, a read-only context preview, and the ability to jump directly back to the original session, along with index management features that support incremental or full rebuilding. Although it currently does not support full-text search of the main content in tool results and has a timeline canvas bug, it still significantly improves developers' efficiency when reviewing historical logs.