chore(xaf_agent_hub): 回退链移出共享 config.json（只留 model_chain），落地 2026-11-08 分层裁定

用户 2026-11-06 拍板 + 2026-11-08 落地：fallback_model / default_fallback_model 属机器级回退链，
只写 config.local.json（gitignored），不得写入共享入库的 config.json；config.json 只放 model_chain。
本次移除 6 实例 fallback_model + 顶层 default_fallback_model；lan-248 链第三档校正为 agnes-ai/agnes-3.0-flash
（实测该节点 agnes 前缀为 agnes-ai/，原 agnes/ 不可命中）。

tools/xaf_agent_hub/config.json

@@ -51,7 +51,6 @@
         "E:\\cfc"
       ],
       "default_directory": "E:\\cfc",
-      "fallback_model": "axonhub/deepseek",
       "model_chain": [
         "opencode/deepseek-v4.1-flash-free",
         "axonhub/deepseek",
@@ -69,12 +68,11 @@
         "/sc-data/cfc"
       ],
       "default_directory": "/sc-data/cfc",
-      "fallback_model": "axonhub/deepseek",
       "concurrency_limit": 4,
       "model_chain": [
         "opencode/deepseek-v4.1-flash-free",
         "axonhub/deepseek",
-        "agnes/agnes-3.0-flash"
+        "agnes-ai/agnes-3.0-flash"
       ]
     },
     {
@@ -86,7 +84,6 @@
         "D:\\cfc"
       ],
       "default_directory": "D:\\cfc",
-      "fallback_model": "axonhub/deepseek",
       "model_chain": [
         "opencode/deepseek-v4.1-flash-free",
         "axonhub/deepseek",
@@ -107,7 +104,6 @@
         "C:\\code\\cfc"
       ],
       "default_directory": "C:\\code\\cfc",
-      "fallback_model": "axon/deepseek",
       "concurrency_limit": 4,
       "model_chain": [
         "opencode/deepseek-v4.1-flash-free",
@@ -130,7 +126,6 @@
       "default_directory": "/root/cfc",
       "danger": true,
       "readonly_default": true,
-      "fallback_model": "axonhub/deepseek",
       "model_chain": [
         "opencode/deepseek-v4.1-flash-free",
         "etotem/deepseek",
@@ -149,7 +144,6 @@
         "/sc-data/caremate"
       ],
       "default_directory": "/sc-data/cfc",
-      "fallback_model": "etotem/deepseek",
       "model_chain": [
         "opencode/deepseek-v4.1-flash-free",
         "axonhub/deepseek",
@@ -158,6 +152,5 @@
       "concurrency_limit": 6
     }
   ],
-  "default_dispatch_model": "opencode/deepseek-v4.1-flash-free",
-  "default_fallback_model": "axonhub/deepseek"
-}
+  "default_dispatch_model": "opencode/deepseek-v4.1-flash-free"
+}