fix(router): pass isGroup from adapter through to messaging group creation

The router hardcoded is_group=0 when auto-creating messaging groups,
causing channel mentions to be misclassified as DMs. The Chat SDK
bridge knows which handler fired (onDirectMessage vs onNewMention)
so thread the signal through InboundMessage → InboundEvent → router.

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

src/channels/adapter.ts

@@ -56,6 +56,8 @@ export interface InboundEvent {
      * See InboundMessage.isMention for the full explanation.
      */
     isMention?: boolean;
+    /** True when the source is a group/channel thread, false for DMs. */
+    isGroup?: boolean;
   };
   replyTo?: DeliveryAddress;
 }
@@ -81,6 +83,8 @@ export interface InboundMessage {
    * router falls back to text-match against agent_group_name.
    */
   isMention?: boolean;
+  /** True when the source is a group/channel thread, false for DMs. */
+  isGroup?: boolean;
 }
 
 /** A file attachment to deliver alongside a message. */