v2 phase 5: pending questions with interactive cards

End-to-end ask_user_question flow:
- Agent MCP tool writes question card to messages_out
- Host delivery creates pending_questions row, delivers as Discord Card with buttons
- Local webhook server receives Gateway INTERACTION_CREATE events
- Acknowledges interaction + updates card to show selected answer
- Routes response back to session DB as system message
- MCP tool poll picks up response and returns to agent

Key fixes:
- Poll loop now skips system messages (reserved for MCP tool responses)
- Gateway listener uses webhookUrl forwarding mode for interaction support
- Button custom_id encodes questionId + option text for self-contained routing

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

src/channels/adapter.ts

@@ -24,6 +24,9 @@ export interface ChannelSetup {
 
   /** Called when the adapter discovers metadata about a conversation. */
   onMetadata(platformId: string, name?: string, isGroup?: boolean): void;
+
+  /** Called when a user clicks a button/action in a card (e.g., ask_user_question response). */
+  onAction(questionId: string, selectedOption: string, userId: string): void;
 }
 
 /** Inbound message from adapter to host. */