fix(queue): restart drain when message enqueued after idle window

After a drain loop empties the queue it deletes the key from
FOLLOWUP_QUEUES.  If a new message arrives at that moment
enqueueFollowupRun creates a fresh queue object with draining:false
but never starts a drain, leaving the message stranded until the
next run completes and calls finalizeWithFollowup.

Fix: persist the most recent runFollowup callback per queue key in
FOLLOWUP_RUN_CALLBACKS (drain.ts).  enqueueFollowupRun now calls
kickFollowupDrainIfIdle after a successful push; if a cached
callback exists and no drain is running it calls scheduleFollowupDrain
to restart immediately.  clearSessionQueues cleans up the callback
cache alongside the queue state.

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

@@ -1,4 +1,5 @@
 import { applyQueueDropPolicy, shouldSkipQueueItem } from "../../../utils/queue-helpers.js";
+import { kickFollowupDrainIfIdle } from "./drain.js";
 import { getExistingFollowupQueue, getFollowupQueue } from "./state.js";
 import type { FollowupRun, QueueDedupeMode, QueueSettings } from "./types.js";
 
@@ -53,6 +54,12 @@ export function enqueueFollowupRun(
   }
 
   queue.items.push(run);
+  // If drain finished and deleted the queue before this item arrived, a new queue
+  // object was created (draining: false) but nobody scheduled a drain for it.
+  // Use the cached callback to restart the drain now.
+  if (!queue.draining) {
+    kickFollowupDrainIfIdle(key);
+  }
   return true;
 }