fix(slack-stream): capture lastStreamPayload into const before null-check + spread

Both tsc and tsgo fail to narrow a mutable let variable through a spread
expression regardless of guards or non-null assertions. The fix: assign the
outer let to a block-scoped const, then check the const !== null in a
separate if. TypeScript always narrows a const through a distinct null check
so the spread is accepted by all compiler variants.

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

@@ -553,11 +553,13 @@ export async function dispatchPreparedSlackMessage(prepared: PreparedSlackMessag
       }
       // Fall back to normal delivery with the full accumulated streamed text
       // so the user receives the complete answer even when stop() fails.
-      // lastStreamPayload is guarded by !== null above; the ! assertion is
-      // required because tsc does not narrow mutable let variables through
-      // spread expressions even with an explicit null guard.
-      if (orphanDeleted && lastStreamPayload !== null && streamedText) {
-        await deliverNormally({ ...lastStreamPayload!, text: streamedText }, finalStream.threadTs);
+      // Use a nested const + if so TypeScript can narrow the type of the
+      // captured const (not the outer mutable let) before the spread.
+      if (orphanDeleted && streamedText) {
+        const fallback = lastStreamPayload;
+        if (fallback !== null) {
+          await deliverNormally({ ...fallback, text: streamedText }, finalStream.threadTs);
+        }
       }
     }
   }