Follow-up to 828fc11b addressing code review feedback:
1. Added LoadTownRoutes() - exported function that walks up to find
town-level routes.jsonl (e.g., ~/gt/.beads/routes.jsonl)
2. Updated buildAllowedPrefixSet to use LoadTownRoutes instead of
LoadRoutes, so it finds routes even when importing from a rig's
local beads directory
3. Added unit tests for buildAllowedPrefixSet covering:
- Primary prefix inclusion
- allowed_prefixes config parsing
- Routes from routes.jsonl
- Missing routes.jsonl handling
- Empty beadsDir handling
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>