fix: wire v2 setup flow — barrel import, registration, verification

- Import channel barrel from src/index.ts so channel skills that
  uncomment lines in src/channels/index.ts actually execute
- Rewrite setup/register.ts to create v2 entities (agent_groups,
  messaging_groups, messaging_group_agents) in data/v2.db instead
  of v1's store/messages.db
- Fix setup/verify.ts to check v2 central DB for registered groups
- Add prominent "MESSAGE DROPPED" warnings in router when no agent
  groups are wired, with actionable guidance

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/db/index.ts

@@ -20,6 +20,7 @@ export {
   createMessagingGroupAgent,
   getMessagingGroupAgents,
   getMessagingGroupAgent,
+  getMessagingGroupAgentByPair,
   updateMessagingGroupAgent,
   deleteMessagingGroupAgent,
 } from './messaging-groups.js';