DeepSeek Harness Message Enhancement Plugin

The DSH (DeepSeek Harness) plugin `hw-cola/dsh-message-enhancer` is designed to enhance the web client experience. Its key features include: adding a vertical index scale above the input box to quickly locate messages and plans; improving the plan approval card style, with support for copying Markdown and editing it directly at the bottom; providing system notifications and an alert sound when a reply is completed; and supporting in-place editing and resending of the last user message. The plugin is built on DSH's native mechanisms and must be installed via `dsh plugin`, taking effect after a restart.