dsh-task-board: DeepSeek Harness Task Board Plugin

`dsh-task-board` is a task board client plugin for DeepSeek Harness Desktop. It injects into the conversation view to add a three-column, drag-and-drop kanban board (Backlog, In Progress, Done) to the conversation header. Users can drag task cards into the execution flow; the system automatically creates a session, streams the results, and moves the card to the Done column after the turn completes, with support for reruns and local persistent storage.