Fix Windows PowerShell bootstrap downloads

[freelw]

Summary

• add TLS 1.2 to the protocols used by Windows PowerShell 5.1 before COS downloads
• restore the process-wide protocol selection after downloading
• surface the URL and underlying exception when a download fails
• add a real Windows PowerShell 5.1 bootstrap test against the pinned COS release

Local validation

• Skill ZIP integrity passed
• SkillHub package contains the updated bootstrap script
• skill-creator quick validation passed
• git diff --check passed