dsh-units: Unit conversion plugin for DeepSeek Harness

`dsh-units` is a lightweight unit conversion plugin for DeepSeek Harness (DSH) designed to address common errors made by agents during conversion. It provides two core tools, `convert_unit` and `list_units`, covering 17 categories including length, mass, temperature, data size, and fuel consumption. It ensures accurate conversion of confusing units such as MB vs. MiB, different definitions of horsepower, and mpg vs. L/100km. Based on a static unit table and performing pure arithmetic calculations, this plugin has high security as it does not involve network access, file system access, or code execution. `convert_unit` supports input via symbols or full names, rejects illegal operations, and returns structured results containing formulas and categories for explanation, while `list_units` is used to query available units. The current version...