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.

scripts/common.sh

@@ -0,0 +1,5 @@
+# Sourced by the other scripts: SSH target of the test VM.
+VM_USER="${VM_USER:-theupriser}"
+VM_PORT="${VM_PORT:-2222}"
+VM_HOST="${VM_HOST:-localhost}"
+vm_ssh() { ssh -p "$VM_PORT" -o BatchMode=yes "$VM_USER@$VM_HOST" "$@"; }