dsh-rectangle: Rectangle Calculation Plugin

DeepSeek Harness (DSH) provides a plugin-based architecture, in which dsh-rectangle is a geometry calculation tool maintained by uckkk. This plugin is implemented with pure Node.js, has no network dependencies at runtime, and requires @deepseek-ai/cordis (^4.0.1) and @deepseek-ai/dsh-tools (>=0.1.0-rc.6). It can be installed with `dsh plugin add github:uckkk/dsh-rectangle`. After installation, the registered tools can be invoked directly in a DSH session to perform rectangle calculations.