fix(history): add LRU eviction for groupHistories to prevent memory leak

Add evictOldHistoryKeys() function that removes oldest keys when the
history map exceeds MAX_HISTORY_KEYS (1000). Called automatically in
appendHistoryEntry() to bound memory growth.

The map previously grew unbounded as users interacted with more groups
over time. Growth is O(unique groups) not O(messages), but still causes
slow memory accumulation on long-running instances.

Fixes #2384

src/auto-reply/reply/history.ts

@@ -3,6 +3,26 @@ import { CURRENT_MESSAGE_MARKER } from "./mentions.js";
 export const HISTORY_CONTEXT_MARKER = "[Chat messages since your last reply - for context]";
 export const DEFAULT_GROUP_HISTORY_LIMIT = 50;
 
+/** Maximum number of group history keys to retain (LRU eviction when exceeded). */
+export const MAX_HISTORY_KEYS = 1000;
+
+/**
+ * Evict oldest keys from a history map when it exceeds MAX_HISTORY_KEYS.
+ * Uses Map's insertion order for LRU-like behavior.
+ */
+export function evictOldHistoryKeys<T>(
+  historyMap: Map<string, T[]>,
+  maxKeys: number = MAX_HISTORY_KEYS,
+): void {
+  if (historyMap.size <= maxKeys) return;
+  const keysToDelete = historyMap.size - maxKeys;
+  const iterator = historyMap.keys();
+  for (let i = 0; i < keysToDelete; i++) {
+    const key = iterator.next().value;
+    if (key !== undefined) historyMap.delete(key);
+  }
+}
+
 export type HistoryEntry = {
   sender: string;
   body: string;
@@ -35,6 +55,8 @@ export function appendHistoryEntry<T extends HistoryEntry>(params: {
   history.push(entry);
   while (history.length > params.limit) history.shift();
   historyMap.set(historyKey, history);
+  // Evict oldest keys if map exceeds max size to prevent unbounded memory growth
+  evictOldHistoryKeys(historyMap);
   return history;
 }