fix(reply-queue): remove the drained item by reference instead of front index

drainNextQueueItem captured items[0], awaited the run, then shift()-ed
index 0 assuming it still held the item it ran. Concurrent inbound
messages mutate the same shared items array, and at or over cap
applyQueueDropPolicy splices items off the front, so a burst arriving
while item[0] is in flight can shift a different, still-undelivered
survivor into index 0. shift() then deletes that survivor: it is never
run and is not counted in the overflow summary, so the agent silently
ignores a message it should have answered.

Remove the item that actually ran by identity via a new
removeQueuedItemsByRef helper, and apply the same reference-based
removal to the collect path in drain.ts, which had the same positional
splice(0, groupItems.length) assumption after an awaited group run.

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

@@ -9,6 +9,7 @@ import {
   drainCollectQueueStep,
   drainNextQueueItem,
   hasCrossChannelItems,
+  removeQueuedItemsByRef,
   previewQueueSummaryPrompt,
   waitForQueueDebounce,
 } from "../../../utils/queue-helpers.js";
@@ -497,7 +498,7 @@ export function scheduleFollowupDrain(
             } else {
               await drainGroup();
             }
-            queue.items.splice(0, groupItems.length);
+            removeQueuedItemsByRef(queue.items, groupItems);
             if (pendingSummary) {
               clearFollowupQueueSummaryState(queue);
               pendingSummary = undefined;

src/utils/queue-helpers.ts

@@ -166,6 +166,15 @@ export function beginQueueDrain<T extends { draining: boolean }>(
   return queue;
 }
 
+export function removeQueuedItemsByRef<T>(items: T[], processed: readonly T[]): void {
+  for (const item of processed) {
+    const idx = items.indexOf(item);
+    if (idx !== -1) {
+      items.splice(idx, 1);
+    }
+  }
+}
+
 /** Run and remove the next queued item, returning false when empty. */
 export async function drainNextQueueItem<T>(
   items: T[],
@@ -176,7 +185,7 @@ export async function drainNextQueueItem<T>(
     return false;
   }
   await run(next);
-  items.shift();
+  removeQueuedItemsByRef(items, [next]);
   return true;
 }