i18n(settings): translate Agents nav label in zh-CN and zh-TW

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/i18n/messages/zh-CN.json

@@ -86,7 +86,7 @@
     "preferences": "偏好设置",
     "nav": {
       "appearance": "外观",
-      "agents": "Agents",
+      "agents": "智能体",
       "mcp": "MCP",
       "skills": "Skills",
       "shortcuts": "快捷键",

src/i18n/messages/zh-TW.json

@@ -86,7 +86,7 @@
     "preferences": "偏好設定",
     "nav": {
       "appearance": "外觀",
-      "agents": "Agents",
+      "agents": "智能體",
       "mcp": "MCP",
       "skills": "Skills",
       "shortcuts": "快捷鍵",