refactor(gateway): introduce the Gateway service ABC (docker impl)

Replace the docker backend's ad-hoc gateway wiring with the shared `Gateway`
service ABC (in the gateway package), the first of the two per-host service
classes that supersede the per-backend infra glue.

The contract is backend-neutral: `connect_to_orchestrator(url, gateway_token)`
binds the gateway to its control plane and brings it up, `address()` reports
the agent-facing proxy target, `ca_cert_pem()` vends the mitmproxy CA, and
`provisioning_transport()` hands back the exec/cp seam git-gate provisioning
stages per-bottle repos + deploy keys through. `GatewayProvisionError` +
`GatewayTransport` move to the ABC so every backend shares them.

Key trust-model change: the gateway no longer mints its own token. It never
holds the signing key (#469), so the orchestrator mints the role-scoped
`gateway` JWT and hands it in via `connect_to_orchestrator`; the gateway only
injects it. `DockerGateway.ensure_running` becomes `connect_to_orchestrator`
(stashing the URL + token as instance state), and the docker launch flow reads
`address()` / `provisioning_transport()` off the service rather than
re-deriving the container IP + transport itself.

macOS + firecracker gateways move under the ABC in following commits.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

bot_bottle/backend/docker/consolidated_launch.py

@@ -61,20 +61,6 @@ def _network_cidr(network: str) -> str:
     return cidr
 
 
-def _container_ip(name: str, network: str) -> str:
-    """A container's IPv4 address on `network`, or raise."""
-    proc = run_docker([
-        "docker", "inspect", "--format",
-        f'{{{{(index .NetworkSettings.Networks "{network}").IPAddress}}}}', name,
-    ])
-    ip = proc.stdout.strip()
-    if proc.returncode != 0 or not ip:
-        raise ConsolidatedLaunchError(
-            f"container {name} has no address on {network}: {proc.stderr.strip()}"
-        )
-    return ip
-
-
 def _network_container_ips(network: str) -> list[str]:
     """Every address currently assigned on the gateway network — the ground
     truth for "in use": the infra container and every live agent. Read from
@@ -157,16 +143,17 @@ def launch_consolidated(
     service = service or DockerInfraService()
     url = service.ensure_running()
     # Agents attribute against the *gateway* container (data plane), not the
-    # orchestrator — the two planes are now separate containers.
-    gateway_name = service.gateway_name
-    _reprovision_running_bottles(url, network=network, infra_name=gateway_name)
+    # orchestrator — the two planes are now separate containers. Read its
+    # agent-facing address + provisioning transport off the Gateway service.
+    gateway = service.gateway()
+    _reprovision_running_bottles(url, network=network, infra_name=gateway.name)
     client = OrchestratorClient(url)
 
     cidr = _network_cidr(network)
-    gateway_ip = _container_ip(gateway_name, network)
+    gateway_ip = gateway.address()
     source_ip = next_free_ip(cidr, _network_container_ips(network))
 
-    transport = DockerGatewayTransport(gateway_name)
+    transport = gateway.provisioning_transport()
     reg = provision_bottle(
         client, source_ip, egress_plan, git_gate_plan, transport,
         image_ref=image_ref, tokens=tokens,