refactor(gateway): each backend's transport in its own gateway_transport.py

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>

tests/unit/test_gateway_provision.py

@@ -8,13 +8,13 @@ from unittest.mock import Mock, patch
 
 from bot_bottle.backend.docker.gateway_provision import (
     GatewayProvisionError,
-    DockerGatewayTransport,
     deprovision_git_gate,
     provision_git_gate,
 )
+from bot_bottle.backend.docker.gateway_transport import DockerGatewayTransport
 from bot_bottle.git_gate import GitGatePlan, GitGateUpstream
 
-_RUN = "bot_bottle.backend.docker.gateway_provision.run_docker"
+_RUN = "bot_bottle.backend.docker.gateway_transport.run_docker"
 
 
 def _proc(returncode: int = 0, stderr: str = "") -> Mock: