dsh-add-conversation: Add a Conversation Reference in One Click

The `dsh-add-conversation` plugin provides a Codex-style quoted conversation feature for the DeepSeek Harness Web GUI without modifying the source code. Users can select text in the chat area and use a floating button to convert it into a staged quote chip, supporting up to 12 chips. When sending a message, the staged content is automatically inserted at the beginning of the message as a Markdown blockquote. The plugin is implemented using a browser-side script, supports up to 8,000 characters of quoted content, and allows closing the preview with Esc. Install it with `dsh plugin --profile web add dsh-add-conversation`.