dsh-nav-pointer: Add a message pointer guide rail to the left side of the DSH Web Chat interface

# Introduction When using the DSH Web client for long conversations, there is a common minor annoyance: wanting to return to a previously sent instruction requires scrolling up screen by screen with the scroll wheel, making it increasingly difficult to locate messages as the number of messages grows. The `dsh-nav-pointer` plugin solves this navigation problem. It adds a message pointer rail to the left side of the chat area, where each user message corresponds to a short horizontal marker. Clicking the marker smoothly scrolls to the corresponding position, and hovering allows you to preview the message content. The following sections introduce the plugin's purpose, features, installation, and configuration. # What is it? The `dsh-nav-pointer` is a DSH (DeepSeek Harness) Web client plugin maintained by kongdexu, licensed under MIT, and currently at version 0.4.0. It works via `shell`