Adds the ChangeThisFile file-conversion skill to Document Processing.

• Repo: https://github.com/aadilr/changethisfile-mcp (MIT)
• Converts files between 999 routes (images, video, audio, documents, data, fonts, ebooks, archives) via the free changethisfile.com API — no API key or signup.
• Skill is MCP-first (changethisfile:convert_file) with a zero-dependency curl script fallback, so it works in Claude Code, Codex CLI, Cursor, Gemini CLI, etc.
• Install: npx skills add aadilr/changethisfile-mcp
• Tested end-to-end (md→pdf, csv→xlsx, error surfaces).