docs: default visible columns contract (v1.0.79)

沉淀 _jiangchang_tables.default_visible_columns_json：技能声明默认展示列；未配置全显；用户改「列」后本机覆盖；与 visible=0 硬隐藏区分。

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/test_docs_standards.py

@@ -344,6 +344,8 @@ class TestDocsStandards(unittest.TestCase):
             "DEFAULT",
             "description",
             "文件 / 图片上传",
+            "default_visible_columns_json",
+            "默认展示列",
         ):
             self.assertIn(marker, text, msg=f"SCHEMA.md missing {marker!r}")