Give every backend a `gateway_transport.py` holding just its `GatewayTransport`
impl, named consistently with the backend: `DockerGatewayTransport`,
`MacosGatewayTransport` (was `AppleGatewayTransport`), and
`FirecrackerGatewayTransport` (was `SshGatewayTransport`).
- docker: split `DockerGatewayTransport` out of `gateway_provision.py`, which
keeps only the backend-neutral provisioning logic (provision_git_gate /
deprovision_git_gate) the other backends share.
- macOS: `gateway_provision.py` held only the transport, so it's replaced by
`gateway_transport.py`.
- firecracker: move the transport out of `gateway.py`.
Behaviour-preserving; importers + tests updated.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
