dsh-turn-collapse: A Codex-Style Hierarchical Collapse Plugin for DeepSeek Harness

`dsh-turn-collapse` is a purely client-side plugin for the DeepSeek Harness (DSH) Web UI, designed to improve the visual experience of conversation flow. It emulates a Codex-like style by folding reasoning and tool calls between visible text outputs into a single unit, thereby reducing interface clutter. The plugin supports collapsed-by-default behavior, state persistence, and reuses DSH design tokens. By using `MutationObserver` to monitor DOM changes, it runs within the browser sandbox and effectively enhances the readability of the DSH Web UI.