Supervisor for gateway-owned ingress children.

Default LemonGateway.Application startup is execution-only; this supervisor starts transport, command, SMS, and voice children only when :gateway_ingress_enabled is set.

These surfaces are gateway-owned by design, not pending migration. SMS is an OTP-code inbox exposed as agent tools with no reply path; webhook is a synchronous automation trigger that must answer in the request cycle that called it; voice holds a live bidirectional media session. None of the three fit LemonChannels.Plugin, whose deliver/1 is fire-and-forget. Email is the one surface slated to move to lemon_channels.

See docs/platform/transport-unification.md for the disposition of each surface and the plugin-contract gaps behind it.