fix: support Windows PowerShell 5.1 bootstrap downloads

[edgar-mission200]

Fixes #195

Windows PowerShell 5.1 can route Invoke-WebRequest through the legacy Internet Explorer parser unless -UseBasicParsing is supplied. This patch adds -UseBasicParsing to both download calls in all three shipped ensure-monk-agent.ps1 copies.

Validation:

• powershell.exe -NoLogo -NoProfile -ExecutionPolicy Bypass -File .\tests\ensure-monk-agent-basic-parsing.ps1
• Result: ensure_basic_parsing_status=pass installers=3 downloads=6

[nooga]

This is a near-identical, later-filed version of #196 (opened the next day, same fix location, same missing CI-wiring gap — install-e2e.yml isn't touched here either). #196 was open first and is the reporter's own PR for the underlying issue, so closing this one in favor of that thread rather than reviewing both in parallel.