dsh-selection-followup: Select DSH Response Text to Follow Up or Copy

`dsh-selection-followup` is a client-side plugin that optimizes the DeepSeek Harness interaction. When a user selects text in a chat reply, a “Follow up” or “Copy” bubble appears below the selection. Clicking “Follow up” inserts the selected content into the input box and focuses it, so the user can continue asking questions; clicking “Copy” copies it directly to the clipboard. The plugin is easy to install, works only in the conversation area, and quotes are capped at 2,000 characters.