dsh-skills-nexus: Universal DSH Skill Adapter

`dsh-skills-nexus` is a universal adapter designed to address the integration gap for content repositories in the DeepSeek Harness (DSH) ecosystem. It allows users to register a GitHub repository containing a `SKILL.md` file folder directly as a DSH skill via symbolic links, without writing plugin code or packaging. The tool provides commands such as `add`, `list`, `update`, and `remove`, supports shell autocompletion, and requires a Node.js >= 20.0.0 environment. It simplifies skill management and improves the efficiency of integrating content repositories.