docs: note USB passthrough requirement for ha1 migration

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docs/plans/host-migration-to-opentofu.md

@@ -158,6 +158,12 @@ For each stateful host, the procedure is:
 coordinator state in `/var/lib/zigbee2mqtt` should preserve pairings, but verify on a
 non-critical time window.
 
+**USB Passthrough:** The ha1 VM has a USB device passed through from the Proxmox hypervisor
+(the Zigbee coordinator). The new VM must be configured with the same USB passthrough in
+OpenTofu/Proxmox. Verify the USB device ID on the hypervisor and add the appropriate
+`usb` block to the VM definition in `terraform/vms.tf`. The USB device must be passed
+through before starting Zigbee2MQTT on the new host.
+
 ## Phase 5: Decommission jump Host
 
 1. Verify nothing depends on the jump host (no SSH proxy configs pointing to it, etc.)