[BUG] `--universal` installs to `.agent/` not `.agents/`

Bug Description A clear and concise description of the bug.

Steps to Reproduce

1. Run openskills install --universal <some skill>

Expected Behavior It should install to the ~/.agents/ (or .agents/) - as per the official agents-skills specification

This is also where Codex and OpenCode (among others) reads skills from

Actual Behavior Skills are installed to ~/.agent/ (or .agent/)

Environment

• openskills version: (openskills --version): 1.5.0
• Operating System: Ubuntu 24.04
• Node.js version: (node --version): v24.14.0
• AI agent: (Claude Code, Cursor, Windsurf, Aider, etc.): OpenCode & Codex

Installation Context

• Global install (npm i -g openskills)
• Local install via npm link
• Installed skills: (list from openskills list)

Additional Context Add any other relevant information, error messages, or screenshots.