fix: don't consume replyPlan reference eagerly for streaming check

The streaming check was calling replyPlan.nextThreadTs() at setup time
to determine if a thread_ts existed, which consumed the first reference
before the deliver callback ran. Use incomingThreadTs/statusThreadTs
directly for the streaming eligibility check instead.

src/slack/monitor/message-handler/dispatch.ts

@@ -138,11 +138,13 @@ export async function dispatchPreparedSlackMessage(prepared: PreparedSlackMessag
   // Slack native text streaming state
   // -----------------------------------------------------------------------
   const streamingEnabled = account.config.streaming === true;
-  const replyThreadTs = replyPlan.nextThreadTs();
+
+  // Peek at the thread target without consuming it (for streaming check only).
+  const streamThreadHint = incomingThreadTs ?? statusThreadTs;
 
   const useStreaming = shouldUseStreaming({
     streamingEnabled,
-    threadTs: replyThreadTs ?? incomingThreadTs ?? statusThreadTs,
+    threadTs: streamThreadHint,
   });
 
   let streamSession: SlackStreamSession | null = null;