The witness manager was using rig-level beads path to look up role
configuration, but role beads use the hq- prefix and live in town-level
beads. This caused "unexpected end of JSON input" errors when starting
witnesses because the rig database (with gt- prefix) couldn't find
hq-witness-role.
Changed roleConfig() to use townRoot instead of rig.BeadsPath() to
correctly resolve town-level role beads.
Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>