@lcthe/dsh-timeline-rail: Add a message timeline for DeepSeek Harness long sessions

@lcthe/dsh-timeline-rail is a DSH web session message timeline navigation plugin (author: lcthe, version 0.2.12, MIT license). It solves the problem of repeatedly scrolling through the message stream to locate user messages in long sessions by rendering a 1px vertical rail on the left edge of the conversation and drawing equally spaced (12px) ticks for every durable user message. Hovering enables message preview (with user messages limited to 2 lines and assistant replies to 3 lines), while clicking aligns the Nth message to the top of the message area. This plugin is a pure presentation client plugin registered to the `conversation.input.dock` slot; it reads the real-time `ConversationSnapshot` via `useSession` and reuses product data attributes.