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/messaging-groups.ts

@@ -71,6 +71,15 @@ export function getMessagingGroupAgents(messagingGroupId: string): MessagingGrou
     .all(messagingGroupId) as MessagingGroupAgent[];
 }
 
+export function getMessagingGroupAgentByPair(
+  messagingGroupId: string,
+  agentGroupId: string,
+): MessagingGroupAgent | undefined {
+  return getDb()
+    .prepare('SELECT * FROM messaging_group_agents WHERE messaging_group_id = ? AND agent_group_id = ?')
+    .get(messagingGroupId, agentGroupId) as MessagingGroupAgent | undefined;
+}
+
 export function getMessagingGroupAgent(id: string): MessagingGroupAgent | undefined {
   return getDb().prepare('SELECT * FROM messaging_group_agents WHERE id = ?').get(id) as
     | MessagingGroupAgent