fix: use basic parsing for Windows agent downloads

[qtcqtc]

Closes #195.

Windows PowerShell 5.1 delegates Invoke-WebRequest response parsing to the Internet Explorer engine unless -UseBasicParsing is explicit. On Server Core and fresh profiles where that engine is unavailable or uninitialized, the native Windows bootstrap fails before it can download either the checksum or monk-agent.exe.

This change:

• adds -UseBasicParsing to the checksum and archive downloads in all three shipped PowerShell installer copies;
• adds a regression harness that simulates the headless PowerShell 5.1 failure;
• exercises the full mocked download, checksum verification, ZIP extraction, install, and output-path flow;
• verifies the generated PowerShell installer copies remain byte-identical.

The regression test fails against v0.1.52 with:

The response content cannot be parsed because the Internet Explorer engine is not available.

After the fix, both Windows PowerShell 5.1 and the current host pass:

ensure_basic_parsing_status=pass downloads=2
Windows block-monk fallback tests passed.

git diff --check also passes.