fix(excel): flush import data to disk and surface write failures

src/officecli/CommandBuilder.Import.cs

@@ -113,6 +113,10 @@ private static Command BuildImportCommand(Option<bool> jsonOption)
             ResidentClient.SendClose(file.FullName);
             using var handler = new OfficeCli.Handlers.ExcelHandler(file.FullName, editable: true);
             var msg = handler.Import(parentPath, csvContent, delimiter, header, startCell);
+            // Import works against ExcelHandler's crash-atomic in-memory package.
+            // Flush before reporting success so write-back failures are surfaced
+            // as command errors instead of being swallowed by best-effort Dispose.
+            handler.Save();
             if (json)
                 Console.WriteLine(OutputFormatter.WrapEnvelopeText(msg));
             else

src/officecli/Handlers/Excel/ExcelHandler.Helpers.Sheet.cs

@@ -197,6 +197,12 @@ private static Worksheet GetSheet(WorksheetPart part) =>
     /// </summary>
     private void SaveWorksheet(WorksheetPart part)
     {
+        // A dirty worksheet is itself proof that the editable package was
+        // modified.  Keep this invariant here instead of relying on every
+        // caller to set Modified separately: Import writes rows directly and
+        // used to enqueue the part without setting Modified, so Dispose()
+        // treated the session as read-only and discarded the in-memory copy.
+        Modified = true;
         _dirtyWorksheets.Add(part);
     }