dsh-skillport: Reusing Existing Agent Skills in DSH

`dsh-skillport` is an MIT-licensed DSH plugin designed to help developers seamlessly integrate skill files from existing tools (such as Claude Code, Cursor, and Gemini CLI) into the DSH native registry, addressing DSH's inability to directly discover or read these adjacent formats. The plugin mainly implements three major features: 1. **Multi-source discovery**: Automatically scans locations not covered by DSH by default, such as `.claude/skills` and `~/.gemini/antigravity/skills`, while avoiding duplicate scanning of DSH's native paths. 2. **Format conversion**: Converts Cursor rules (`.mdc`) into model-invocable skills, and converts Claude Code commands into