Add letter template for YSWS - Calculate

This issue has no description.

The issue requests a letter template for YSWS - Calculate but provides no file path, format, or acceptance criteria. First locate the repository's existing letter templates and any references to YSWS or Calculate, then confirm the expected template structure with a maintainer. Done means the new template is added in the established location and matches the project's existing format.

Written by the indexing model from the issue text.