fix: handle string thread_ts from Slack properly

Issue #4380: The queue drain logic was using typeof === 'number'
to check for thread IDs, which fails for Slack where thread_ts
is a string like '1737766124.329349'.

Changed three occurrences:
- Line 43: typeof threadId !== 'number' → threadId == null
- Line 49: typeof threadId === 'number' → threadId != null
- Line 75: typeof originatingThreadId === 'number' → originatingThreadId != null

This allows Slack string thread IDs to be properly recognized
while still filtering out undefined/null values.

src/auto-reply/reply/queue/drain.ts

@@ -40,13 +40,13 @@ export function scheduleFollowupDrain(
             const to = item.originatingTo;
             const accountId = item.originatingAccountId;
             const threadId = item.originatingThreadId;
-            if (!channel && !to && !accountId && typeof threadId !== "number") {
+            if (!channel && !to && !accountId && threadId == null) {
               return {};
             }
             if (!isRoutableChannel(channel) || !to) {
               return { cross: true };
             }
-            const threadKey = typeof threadId === "number" ? String(threadId) : "";
+            const threadKey = threadId != null ? String(threadId) : "";
             return {
               key: [channel, to, accountId || "", threadKey].join("|"),
             };
@@ -72,7 +72,7 @@ export function scheduleFollowupDrain(
             (i) => i.originatingAccountId,
           )?.originatingAccountId;
           const originatingThreadId = items.find(
-            (i) => typeof i.originatingThreadId === "number",
+            (i) => i.originatingThreadId != null,
           )?.originatingThreadId;
 
           const prompt = buildCollectPrompt({