Define Standard Subject Line for Migration Reminder Emails

**Description**
As part of the migration communications process for re-engaging with clients , the team requires a standardized subject line for reminder emails sent to businesses that have not yet completed affiliation or account setup activities following migration.

**The intent is to ensure**
same basic subject/body structure, with only a small reminder-style tweak, to balance user clarity with minimal implementation effort

**Acceptance Criteria**
Approved reminder email subject line is documented.