live-image: bundle nonfree firmware (i915 GuC et al.)

Tiger Lake and later Intel iGPUs need i915/tgl_guc_*.bin; without
it the i915 init wedges and floods the console. Same story on most
modern wifi/NIC hardware. Pull firmware-linux-nonfree (metapackage
covering misc-nonfree, iwlwifi, realtek, amd-graphics, …) from the
bookworm non-free-firmware repo — single line fix, ~500MB cost to
the squashfs, worth it for booting arbitrary repaired hosts.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

live-image/mkosi.conf

@@ -8,7 +8,11 @@
 [Distribution]
 Distribution=debian
 Release=bookworm
-Repositories=main
+# non-free-firmware is where bookworm landed i915 GuC/HuC, iwlwifi,
+# amdgpu, nvidia-*, realtek NIC firmware, etc. — anything we'd want
+# when PXE-booting a random repaired host. Without it i915 wedges
+# on Tiger Lake+ and drags the serial console with it.
+Repositories=main non-free-firmware
 
 [Output]
 Format=directory
@@ -35,6 +39,7 @@ Packages=
     dmidecode
     pciutils
     usbutils
+    firmware-linux-nonfree
 
 # Phase 4 will add: smartmontools stress-ng fio iperf3 lshw lm-sensors