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.

share/guest-ssh-setup.sh

@@ -0,0 +1,31 @@
+#!/bin/bash
+# Run INSIDE the VM (as your normal user) to make it reachable from the host
+# over SSH with key auth and passwordless sudo:
+#   sudo mount -t 9p -o trans=virtio,version=9p2000.L vmtools /media && /media/guest-ssh-setup.sh
+set -euo pipefail
+here="$(cd "$(dirname "$0")" && pwd)"
+
+sudo pacman -S --needed --noconfirm openssh
+sudo systemctl enable --now sshd
+
+# Open port 22 if a firewall is active.
+if command -v ufw >/dev/null && sudo ufw status | grep -q 'Status: active'; then
+    sudo ufw allow ssh
+fi
+if command -v firewall-cmd >/dev/null && sudo firewall-cmd --state >/dev/null 2>&1; then
+    sudo firewall-cmd --permanent --add-service=ssh && sudo firewall-cmd --reload
+fi
+
+# Authorize the host's key(s), without duplicates.
+install -d -m 700 ~/.ssh
+touch ~/.ssh/authorized_keys && chmod 600 ~/.ssh/authorized_keys
+while IFS= read -r key; do
+    [[ -n "$key" ]] && ! grep -qxF "$key" ~/.ssh/authorized_keys && echo "$key" >> ~/.ssh/authorized_keys
+done < <(cat "$here/host-keys.pub" 2>/dev/null)
+
+# Test VM only: passwordless sudo so the host can drive it non-interactively.
+echo "$USER ALL=(ALL) NOPASSWD: ALL" | sudo tee /etc/sudoers.d/99-test-vm >/dev/null
+sudo chmod 440 /etc/sudoers.d/99-test-vm
+
+echo "Done. From the host: ssh -p 2222 $USER@localhost"
+[[ -s "$here/host-keys.pub" ]] || echo "No host keys were shared; add yours from the host (see add-ssh-key.ps1 on Windows)."