refactor: replace skill code with skill ID across the application

- Updated skill handling to use skill IDs instead of skill codes in various components, services, and models.
- Modified tests to reflect changes in skill identification.
- Removed references to skill codes in favor of skill IDs for consistency and clarity.
- Updated localization files to remove skill code references and adjust error messages accordingly.

web/messages/en-US.json

@@ -1612,7 +1612,6 @@
     "refresh": "Refresh",
     "new": "New",
     "filterName": "Filter by name",
-    "filterCode": "Filter by code",
     "searchStatus": "Search statuses",
     "emptyStatus": "No matching statuses",
     "query": "Search",
@@ -1621,8 +1620,6 @@
     "actions": "Actions",
     "loadingRows": "Loading skills...",
     "emptyRows": "No matching skills",
-    "codeRequired": "Enter a skill code.",
-    "codeInvalid": "Skill codes can contain letters, numbers, underscores, and hyphens only.",
     "nameRequired": "Enter a skill name.",
     "instructionRequired": "Enter skill instructions.",
     "editTitle": "Edit",
@@ -1632,8 +1629,6 @@
     "save": "Save",
     "create": "Create",
     "loading": "Loading...",
-    "code": "Code",
-    "codePlaceholder": "Example: refund_skill",
     "name": "Name",
     "namePlaceholder": "Example: Refund Handling",
     "description": "Description",

web/messages/zh-CN.json

@@ -1613,7 +1613,6 @@
     "refresh": "刷新",
     "new": "新建",
     "filterName": "按名称筛选",
-    "filterCode": "按编码筛选",
     "searchStatus": "搜索状态",
     "emptyStatus": "未找到状态",
     "query": "查询",
@@ -1622,8 +1621,6 @@
     "actions": "操作",
     "loadingRows": "正在加载 Skill...",
     "emptyRows": "没有匹配的 Skill",
-    "codeRequired": "Skill 编码不能为空",
-    "codeInvalid": "Skill 编码仅支持字母、数字、下划线和中划线",
     "nameRequired": "Skill 名称不能为空",
     "instructionRequired": "技能说明不能为空",
     "editTitle": "编辑",
@@ -1633,8 +1630,6 @@
     "save": "保存",
     "create": "创建",
     "loading": "加载中...",
-    "code": "编码",
-    "codePlaceholder": "例如：refund_skill",
     "name": "名称",
     "namePlaceholder": "例如：退款处理",
     "description": "描述",