dsh-performance-slider: DeepSeek Harness's Codex-style 6-level Performance Slider

`dsh-performance-slider` is a performance-switching plugin for DeepSeek Harness that replicates the desktop experience. It uses a 1–6 level slider to synchronously switch models, reasoning levels, and 31 character background images, simplifying manual parameter tuning. The plugin supports automatic installation of the cordis patch and provides a standalone preview feature. User selections are automatically saved to local storage and can be restored across ports.