dsh-tick-rail: DSH Web Session Tick Navigation

dsh-tick-rail is a pure client-side plugin for the DeepSeek Harness (DSH) Web UI, designed to address the difficulty of locating specific messages in long sessions. The plugin generates a tick rail on the session sidebar, using user messages as an index for quick navigation and previewing. Core features include: each user message corresponds to a tick mark, with assistant replies not occupying space; highlighting peaks and displaying message snippets on mouse hover; and support for click-based smooth scrolling along with full keyboard controls (arrow keys for movement, Enter/Space to jump, and Escape to collapse). The plugin automatically hides when there are fewer than three user messages or when no session is open. As an open-source project under the MIT license, it can be installed from local sources or GitHub using the `dsh plugin add` command, requiring a restart of `dsh web` to take effect.