[BUG] `subpath` uses backslash on Windows

[gdm257]

Bug Description
<skill-root>/.openskills.json generated by openskills install uses double backslashes \\ as path separator on Windows e.g. "subpath": "plugins\\ui-design\\skills\\accessibility-compliance"

Steps to Reproduce

1. openskills install -g ...
2. cat ~/.claude/skills/<skill>/.openskills.json | grep subpath

Expected Behavior
Use / in subpath

Actual Behavior
\\ is used

Environment

• openskills version: 1.5.0
• Operating System: Windows 10
• Node.js version: v24.13.0
• AI agent: Claude Code

Installation Context

• Global install (npm i -g openskills)
• Local install via npm link
• Installed skills: any skill e.g. anthropics/claude-plugins-official

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