dsh-message-map: Porting a Codex-Style Conversation Message Navigation Track to DeepSeek Harness

`dsh-message-map` is a Codex-style navigation plugin ported for DeepSeek Harness (DSH). It draws a vertical ruler on the left side of the chat area to help users efficiently browse long conversations. Features include automatically displaying the ruler, showing summaries on hover, jumping to a message with a click, scrolling by dragging, and automatically responding to message changes. It is easy to install, requires no special permissions, only listens to rendered nodes, and does not modify data. It is ideal for developers who frequently need to locate context, significantly improving interaction efficiency in long conversations.