Fix Windows install.ps1 constructing incorrect release artifact filename

[Copilot]

The GitHub release tag (v0.3.0) includes a v prefix, but release artifact filenames do not (prr_0.3.0_windows_amd64.zip). install.ps1 was using the raw tag name in the filename, producing a 404.

Changes

• scripts/install.ps1: Strip the v prefix before constructing the archive filename, mirroring the existing pattern in install.sh:

# Before
$Archive = "${Binary}_${Version}_windows_${Arch}.zip"
# → prr_v0.3.0_windows_amd64.zip ❌

# After
$VersionNum = $Version -replace '^v', ''
$Archive = "${Binary}_${VersionNum}_windows_${Arch}.zip"
# → prr_0.3.0_windows_amd64.zip ✓

The download URL path still uses the full tag ($Version) — only the filename component is affected.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Windows install.ps1 gets release artifact URL wrong</issue_title>
<issue_description>It is attempting to download:
https://github.com/richardthombs/prr/releases/download/v0.3.0/prr_v0.3.0_windows_amd64.zip

But the release artifact is actually
https://github.com/richardthombs/prr/releases/download/v0.3.0/prr_0.3.0_windows_amd64.zip

Note that there is no "v" in front of the version number in the filename (but there is in the release path)</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Windows install.ps1 gets release artifact URL wrong #26

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