refactor(openclaw): make primary model read-only in Agents panel

The primary model field in the Agents defaults panel now displays as
read-only, eliminating the duplicate edit entry with the "Set as Default
Model" button on provider cards. Fallback models and runtime parameters
remain editable.

src/i18n/locales/en.json

@@ -1218,7 +1218,8 @@
       "description": "Manage agents.defaults in openclaw.json (default model, runtime parameters, etc.)",
       "modelSection": "Model Configuration",
       "primaryModel": "Primary Model",
-      "primaryModelHint": "Format: provider/model-id",
+      "primaryModelHint": "Set via the \"Set as Default Model\" button in the provider list",
+      "notSet": "Not set",
       "fallbackModels": "Fallback Models",
       "fallbackModelsHint": "Comma-separated, ordered by priority",
       "runtimeSection": "Runtime Parameters",

src/i18n/locales/ja.json

@@ -1218,7 +1218,8 @@
       "description": "openclaw.json の agents.defaults 設定を管理（デフォルトモデル、ランタイムパラメータなど）",
       "modelSection": "モデル設定",
       "primaryModel": "プライマリモデル",
-      "primaryModelHint": "形式: provider/model-id",
+      "primaryModelHint": "プロバイダ一覧の「デフォルトモデルに設定」ボタンで設定します",
+      "notSet": "未設定",
       "fallbackModels": "フォールバックモデル",
       "fallbackModelsHint": "カンマ区切り、優先度順",
       "runtimeSection": "ランタイムパラメータ",

src/i18n/locales/zh.json

@@ -1218,7 +1218,8 @@
       "description": "管理 openclaw.json 中的 agents.defaults 配置（默认模型、运行参数等）",
       "modelSection": "模型配置",
       "primaryModel": "主模型",
-      "primaryModelHint": "格式：provider/model-id",
+      "primaryModelHint": "在供应商列表中通过「设为默认模型」按钮设置",
+      "notSet": "未设置",
       "fallbackModels": "回退模型",
       "fallbackModelsHint": "逗号分隔，按优先级排列",
       "runtimeSection": "运行参数",