Update plugins.json

Author: Soulter

plugins.json

@@ -1658,7 +1658,8 @@
   "astrbot_plugin_mnemosyne": {
     "desc": "一个长期记忆插件，基于milvus数据库实现，详情请见README.md",
     "author": "lxfight",
-    "repo": "https://github.com/lxfight/astrbot_plugin_mnemosyne"
+    "repo": "https://github.com/lxfight/astrbot_plugin_mnemosyne",
+    "pinned": true
   },
   "FateTrial_img_identify": {
     "desc": "智谱AI图像识别插件",