docs: fix agents.defaults model configuration format

Change incorrect object format model.primary/fallbacks to correct
flat format model_name/model_fallbacks in Agent defaults example.

The AgentDefaults struct does not support the object format used
in AgentConfig, so the documentation example was misleading.

docs/guides/providers.md

@@ -393,10 +393,8 @@ It also applies cooldown tracking per candidate to avoid immediately retrying a
   ],
   "agents": {
     "defaults": {
-      "model": {
-        "primary": "qwen-main",
-        "fallbacks": ["deepseek-backup", "gemini-backup"]
-      }
+      "model_name": "qwen-main",
+      "model_fallbacks": ["deepseek-backup", "gemini-backup"]
     }
   }
 }

docs/guides/providers.zh.md

@@ -362,10 +362,8 @@ PicoClaw 按下面的规则解析 `provider` 和最终发给上游的模型 ID
   ],
   "agents": {
     "defaults": {
-      "model": {
-        "primary": "qwen-main",
-        "fallbacks": ["deepseek-backup", "gemini-backup"]
-      }
+      "model_name": "qwen-main",
+      "model_fallbacks": ["deepseek-backup", "gemini-backup"]
     }
   }
 }