NestTemplateMigration converts the legacy NestTemplatePath template into
the nest defaults file when the template exists and no defaults file does,
and says whether to clear the setting (converted or nothing to convert)
or the warning to show when the conversion failed, which keeps the
setting. A settings-map overload clears it to the empty text WinForms
stores, so it runs once. WinForms' MainWindowSettings now calls it and
still shows the same warning.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
