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/hooks/message-sent.ts

@@ -23,6 +23,9 @@ type MessageSentDeps = {
     content: string,
     logger: { info: (m: string) => void; warn: (m: string) => void },
   ) => Promise<unknown>;
+  discussionService?: {
+    isClosedDiscussion: (channelId: string) => boolean;
+  };
 };
 
 export function registerMessageSentHook(deps: MessageSentDeps): void {
@@ -36,6 +39,7 @@ export function registerMessageSentHook(deps: MessageSentDeps): void {
     ensurePolicyStateLoaded,
     resolveDiscordUserId,
     sendModeratorMessage,
+    discussionService,
   } = deps;
 
   api.on("message_sent", async (event, ctx) => {
@@ -97,6 +101,14 @@ export function registerMessageSentHook(deps: MessageSentDeps): void {
       }
 
       if (wasNoReply || hasEndSymbol) {
+        // Check if this is a closed discussion channel
+        if (discussionService?.isClosedDiscussion(channelId)) {
+          api.logger.info(
+            `dirigent: message_sent skipping turn advance for closed discussion channel=${channelId} from=${accountId}`,
+          );
+          return;
+        }
+        
         const nextSpeaker = onSpeakerDone(channelId, accountId, wasNoReply);
         const trigger = wasNoReply ? "no_reply_keyword" : "end_symbol";
         const noReplyKeyword = wasNoReply ? (/^NO$/i.test(trimmed) ? "NO" : "NO_REPLY") : "";