Boot KVM guests via UEFI by default; per-image firmware setting

Run 5 on the real host bisected the guest hang: the Debian 13 image
hangs after GRUB under SeaBIOS with or without our seed and with either
CPU model, and boots under OVMF. The image's legacy boot chain is the
problem, not the provider.

images.<key>.firmware is "uefi" (default) or "bios". The domain XML
emits <os firmware="efi"> with secure boot off, so libvirt selects the
OVMF binary and manages the NVRAM file, which the existing nvram undefine
flag removes. Direct-URL images default to UEFI too. OVMF joins the
hypervisor prerequisites; sample, config table, known gaps, decision log
and the execution log record the finding.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

config/samples/providers-config.yaml

@@ -40,6 +40,8 @@ providers:
         debian-13:
           url: https://cloud.debian.org/images/cloud/trixie/latest/debian-13-generic-amd64.qcow2
           # sha256: "<64 hex>"        # optional; verified after download
+          # firmware: uefi            # default; "bios" only for images with no EFI partition
+          #                           # (Debian 13 does not boot via BIOS, see docs/kvm-provider.md)
         ubuntu-24.04:
           url: https://cloud-images.ubuntu.com/noble/current/noble-server-cloudimg-amd64.img
       machineTypes:                   # placement.machineType names a key here