v2 phase 4: channel adapter interface, registry, and host wiring

ChannelAdapter interface with setup/deliver/teardown/setTyping lifecycle.
Self-registration pattern via channel-registry. Host wiring in index-v2
bridges inbound messages to routeInbound and outbound delivery to adapters.

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

src/db/index.ts

@@ -14,6 +14,7 @@ export {
   getMessagingGroup,
   getMessagingGroupByPlatform,
   getAllMessagingGroups,
+  getMessagingGroupsByChannel,
   updateMessagingGroup,
   deleteMessagingGroup,
   createMessagingGroupAgent,