fix: use junction type on Windows for skills symlinks

[ShawInnes]

Changes

Update skillsInstall.ts to use junction-type symlinks for directories on Windows to avoid EPERM errors.

Details

• Check if the symlink target is a directory using fs.statSync()
• Use 'junction' type for directory symlinks on Windows (process.platform === 'win32')
• Junctions don't require elevated privileges or Developer Mode, unlike regular directory symlinks
• Other platforms and file symlinks continue to use default symlink behavior