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.

add-ssh-key.ps1

@@ -0,0 +1,9 @@
+# Windows: authorize your SSH key in the VM (asks for the VM password once).
+# Run after guest-ssh-setup.sh inside the VM, or at least after sshd is enabled there.
+#   .\add-ssh-key.ps1 -User <vm-user>
+param([Parameter(Mandatory)][string]$User)
+$ErrorActionPreference = 'Stop'
+$key = Join-Path $env:USERPROFILE '.ssh\id_ed25519.pub'
+if (-not (Test-Path $key)) { ssh-keygen -t ed25519 -f ($key -replace '\.pub$', '') -N '""' }
+Get-Content $key | ssh -p 2222 "$User@localhost" 'install -d -m 700 ~/.ssh && k=$(cat) && (grep -qxF "$k" ~/.ssh/authorized_keys 2>/dev/null || echo "$k" >> ~/.ssh/authorized_keys) && chmod 600 ~/.ssh/authorized_keys'
+Write-Host "Done. Connect with: ssh -p 2222 $User@localhost"