When importing issues in a multi-rig setup (Gas Town), the prefix
validation was failing for issues with prefixes from other rigs
(e.g., hq-* prefixes from town-level beads).
This fix extends buildAllowedPrefixSet to also load prefixes from
routes.jsonl, allowing issues from any routed rig to pass validation.
Fixes: gt-2maz79
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>