#645 Tenant Welcome Workflow

_system.tenant.welcome workflow sends welcome email after onboarding. Workflow runs on _platform tenant, internal queue. Triggered after _system.tenant.onboard completes. Steps: 1. internal.tenant.send_welcome_email - Send welcome email (internally uses tools.email.send with priority=high) 2. Log onboarding completion to audit Input: tenant_id, admin_email Parent: #639