docs: add work log for 2026-06-15 and update DB deletion policy in README

scratch/peek_excel.cjs

@@ -0,0 +1,6 @@
+const XLSX = require('xlsx');
+const workbook = XLSX.readFile('system_User (20260615).xlsx');
+const sheetName = workbook.SheetNames[0];
+const sheet = workbook.Sheets[sheetName];
+const data = XLSX.utils.sheet_to_json(sheet, { header: 1 });
+console.log(JSON.stringify(data.slice(0, 5), null, 2));