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.sh

@@ -0,0 +1,29 @@
+#!/bin/bash
+# Download the latest CachyOS desktop ISO to cachyos.iso and verify its checksum.
+# Re-running resumes a partial download; an up-to-date ISO is left alone.
+set -euo pipefail
+cd "$(dirname "$0")"
+
+mirror="${MIRROR:-https://nl.mirror.cx/cachyos}"
+
+# The download page embeds the current ISO URL; take the release from it.
+release="$(curl -fsL https://cachyos.org/download/ |
+    grep -oE 'ISO/desktop/[0-9]+/cachyos-desktop-linux-[0-9]+\.iso' | head -n 1)"
+if [[ -z "$release" ]]; then
+    echo "Could not find the current ISO on cachyos.org/download" >&2
+    exit 1
+fi
+url="$mirror/$release"
+name="$(basename "$release")"
+echo "Latest release: $name"
+
+if [[ -f cachyos.iso && "$(cat cachyos.iso.version 2>/dev/null)" != "$name" ]]; then
+    echo "Existing cachyos.iso is an older release, replacing it."
+    rm -f cachyos.iso
+fi
+
+curl -fL -C - -o cachyos.iso "$url"
+echo "$name" > cachyos.iso.version
+
+echo "Verifying checksum..."
+curl -fsL "$url.sha256" | awk '{print $1"  cachyos.iso"}' | sha256sum -c