dsh-msgrail: Message Indexing Rail Plugin for DeepSeek Harness

`dsh-msgrail` is an orbital plugin for DeepSeek Harness designed to quickly locate context in long conversations. It displays a message index as a vertical column of dots along the right edge of the chat area, supporting hover previews, click-to-jump, and automatic history loading. The plugin does not modify host code, adapts to DSH theme skins, and highlights the latest message. It is suitable for developers who frequently switch contexts, but may experience loading delays or layout issues with extremely long histories or narrow windows.