Name the upstream cause of the Debian 13 BIOS hang: Debian bug #1111240

The known-gaps entry, decision log, sample comment and builder comment
blamed the host; the cause is trixie's GRUB config ("terminal_output
gfxterm serial" with a serial console and no video device), reported
and reproduced upstream and still open. UEFI remains the default for the
reason recorded; bookworm images are noted as unaffected.

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

config/samples/providers-config.yaml

@@ -41,7 +41,7 @@ providers:
           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)
+          #                           # (Debian 13 hangs in GRUB under BIOS headless: Debian bug #1111240)
         ubuntu-24.04:
           url: https://cloud-images.ubuntu.com/noble/current/noble-server-cloudimg-amd64.img
       machineTypes:                   # placement.machineType names a key here