Nabu Casa's prebuilt OpenThread RCP image for the ZBT-2
(zbt2_openthread_rcp_2.7.2.0_GitHub-fb0446f53_gsdk_2025.6.2.gbl)
runs at 460800 baud, not the module's 115200 default. Aligns the
radio URL with the firmware so otbr-agent can actually open the
Spinel link after rebuild.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>