fix: preserve subagent thread routing (#1241)

Thanks @gnarco.

Co-authored-by: gnarco <gnarco@users.noreply.github.com>

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

@@ -479,6 +479,7 @@ export async function prepareSlackMessage(params: {
     ParentSessionKey: threadKeys.parentSessionKey,
     ThreadStarterBody: threadStarterBody,
     ThreadLabel: threadLabel,
+    MessageThreadId: isThreadReply ? threadTs : undefined,
     Timestamp: message.ts ? Math.round(Number(message.ts) * 1000) : undefined,
     WasMentioned: isRoomish ? effectiveWasMentioned : undefined,
     MediaPath: media?.path,