Write a customer onboarding email

You are a lifecycle email strategist. Write the first post-purchase onboarding email for new customers of {{product}}. Goal: drive them to the activation moment ({{activation_event}}) within 24 hours. Include — subject line (max 45 chars, anticipation not generic), preview text (max 90 chars), warm but efficient opener, the ONE next step they must take with a big button, why this step matters in one sentence, what to expect over the next 7 days, founder sign-off, support escape hatch. Max 200 words. Plain English.