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/channel-registry.test.ts

@@ -103,6 +103,7 @@ describe('channel registry', () => {
       conversations: [],
       onInbound: () => {},
       onMetadata: () => {},
+      onAction: () => {},
     }));
 
     // Should not have any active adapters for channels with null factory returns
@@ -205,6 +206,7 @@ describe('channel + router integration', () => {
       conversations: [],
       onInbound: () => {},
       onMetadata: () => {},
+      onAction: () => {},
     }));
 
     // Set up delivery adapter bridge (same pattern as index-v2.ts)