docs(sheets): sync chart properties schema (position/size required)

Regenerate flag-schemas.json from upstream sheet-skill-spec: the chart
properties schema now marks position and size as required, and the chart
reference doc reflects the same. flag-schemas.json is print-schema-only
(no client-side validation), so this is a generated-artifact + doc sync
with no CLI behavior change.

shortcuts/sheets/data/flag-schemas.json

@@ -1759,6 +1759,10 @@
             }
           }
         },
+        "required": [
+          "position",
+          "size"
+        ],
         "additionalProperties": {}
       }
     },
@@ -2785,6 +2789,10 @@
             }
           }
         },
+        "required": [
+          "position",
+          "size"
+        ],
         "additionalProperties": {}
       }
     },

skills/lark-sheets/references/lark-sheets-chart.md

@@ -147,9 +147,9 @@ _公共四件套 · 系统：`--yes`、`--dry-run`_
 _创建/更新的图表属性_
 
 **顶层字段**：
-- `position` (object?) — 必填 { row: number, col: string }
+- `position` (object) — 必填 { row: number, col: string }
 - `offset` (object?) — 可选 { row_offset?: number, col_offset?: number }
-- `size` (object?) — 必填 { width: number, height: number }
+- `size` (object) — 必填 { width: number, height: number }
 - `snapshot` (object?) — 图表快照配置 { title?: object, subTitle?: object, style?: object, legend?: oneOf, plotArea?: object, …共 6 项 }
 
 ## Examples