dsh-catalan: Catalan Number Calculation Tool

The `dsh-catalan` plugin for DeepSeek Harness (DSH) is implemented in pure Node.js and is specifically designed for combinatorial computations involving Catalan numbers. This plugin has no network dependencies and, after installing it with the `dsh plugin add` command, its tools can be invoked directly within a session. It relies on relevant DeepSeek core libraries and is licensed under the MIT License.