fix: clear setgid bit from Windows VM mount sources

[SilentTearsDev]

Problem

The Windows VM shared directories can end up with the setgid bit set after running the VM.

For example:

~/Windows -> 2700

The Windows VM permission validation expects 700, causing subsequent VM starts to fail until the setgid bit is manually removed with:

chmod g-s ~/Windows

Fix

Use an explicit 00700 mode when hardening the Windows VM mount source directories so that special permission bits such as setgid are cleared.

Also provide a useful error message when the resulting permissions are unexpected.

Testing

bash -n bin/omarchy-windows-vm
git diff --check

• Reproduced the issue with ~/Windows having mode 2700
• Verified that 00700 clears the setgid bit and results in 700