fix: Add CachyOS test VM dev environment and VM testing skill

QEMU/KVM scripts for a CachyOS KDE test VM (run.sh/run.ps1, get-iso,
guest SSH setup), host helper scripts to drive and inspect the
cachyos-gamescope-boot wizard in the guest, and a Claude Code skill
documenting the test workflow.

get-iso.ps1

@@ -0,0 +1,28 @@
+# Download the latest CachyOS desktop ISO to cachyos.iso and verify its checksum.
+# Windows version of get-iso.sh; re-running resumes a partial download.
+$ErrorActionPreference = 'Stop'
+Set-Location $PSScriptRoot
+
+$mirror = if ($env:MIRROR) { $env:MIRROR } else { 'https://nl.mirror.cx/cachyos' }
+
+$page = (Invoke-WebRequest -UseBasicParsing https://cachyos.org/download/).Content
+$m = [regex]::Match($page, 'ISO/desktop/[0-9]+/cachyos-desktop-linux-[0-9]+\.iso')
+if (-not $m.Success) { throw 'Could not find the current ISO on cachyos.org/download' }
+$url = "$mirror/$($m.Value)"
+$name = Split-Path $m.Value -Leaf
+Write-Host "Latest release: $name"
+
+if ((Test-Path cachyos.iso) -and ((Get-Content cachyos.iso.version -ErrorAction SilentlyContinue) -ne $name)) {
+    Write-Host 'Existing cachyos.iso is an older release, replacing it.'
+    Remove-Item cachyos.iso
+}
+
+# curl.exe ships with Windows 10+ and supports resuming (-C -).
+curl.exe -fL -C - -o cachyos.iso $url
+if ($LASTEXITCODE -ne 0) { throw 'Download failed' }
+Set-Content cachyos.iso.version $name
+
+Write-Host 'Verifying checksum...'
+$expected = ((curl.exe -fsL "$url.sha256") -split '\s+')[0]
+$actual = (Get-FileHash cachyos.iso -Algorithm SHA256).Hash
+if ($actual -ieq $expected) { Write-Host 'cachyos.iso: OK' } else { throw "Checksum mismatch: $actual" }