Complete A6 and A7 tasks: Implement closed discussion channel protections and update tasklist

- Complete tasks A6.1, A6.2, A6.3: Turn manager discussion mode handling
- Complete tasks A7.1, A7.2, A7.3, A7.5: Hook-level discussion channel protections
- Add closed discussion channel check to message-sent hook to prevent handoffs
- Update TASKLIST.md to mark completed tasks with [x]

plugin/index.ts

@@ -226,6 +226,7 @@ export default {
       ensurePolicyStateLoaded,
       resolveDiscordUserId,
       sendModeratorMessage,
+      discussionService,
     });
   },
 };