otbr installed socat from apt at every start, which the namespace
egress baseline denies, so it crash-looped. otbr-agent now speaks
spinel+hdlc+unix to a socket served by a native alpine/socat sidecar
that bridges to slzb-thread:6638 (openthread/otbr image of 2026-11-07;
Unix stream sockets landed in OpenThread #13785). otbr-egress allows
only that TCP link (S28-48).
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>