Harden PowerShell runtime deployment

[jsilverdev]

Summary

• replace PowerShell provider-based runtime copies with .NET directory/file I/O
• preserve valid parent reparse points such as OneDrive redirected Documents/PowerShell
• replace legacy reparse points only at the exact runtime files managed by deploy-pwsh.ps1
• add CI fixtures reproducing broken legacy symlinks for env.ps1, helpers.ps1, aliases.ps1, and the PowerShell profile
• assert the deployed runtime files are regular files under both normal and AllSigned execution policies

This fixes migration failures seen on machines with legacy dotfiles symlinks and OneDrive-redirected PowerShell profile directories.