dsh-message-jump: Enhanced Conversation Navigation and History Traversal

The DeepSeek Harness plugin `dsh-message-jump` solves the challenges of locating and backfilling in long conversations with a Codex-style command navigation bar and terminal-style keyboard navigation. It supports scroll-linked positioning, hover preview, click-to-jump, and full-history coverage, and provides fast recall of historical commands with the ↑/↓ keys, draft protection, and smart yielding. Data is fetched from local session snapshots and RPC calls without relying on private APIs, and it supports customizable styles and behaviors to improve message browsing and input efficiency on the Web.