#927 Email via mail-api instead of SMTP

EARS SPEC: - tools.email.send and all API email paths shall send through mail-api POST /api/v1/emails with an idempotency_key derived from run and task identity. - The codebase shall not import smtplib. - When a durable retry re-sends, mail-api shall receive the same idempotency_key. Parent epic: #913. Synthesis (technical problems / solution domains / alternatives) recorded as a comment when started.