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>

tests/unit/test_consolidated_launch.py

@@ -46,8 +46,12 @@ class TestLaunchConsolidated(unittest.TestCase):
     ):
         service = MagicMock()
         service.ensure_running.return_value = "http://orch:8080"
+        # The launch flow reads the gateway's agent-facing address + provisioning
+        # transport off the Gateway service (service.gateway()).
+        gateway = service.gateway.return_value
+        gateway.name = "bot-bottle-gateway"
+        gateway.address.return_value = "172.18.0.2"
         with patch(f"{_MOD}._network_cidr", return_value="172.18.0.0/16"), \
-                patch(f"{_MOD}._container_ip", return_value="172.18.0.2"), \
                 patch(f"{_MOD}._network_container_ips", return_value=list(on_network)), \
                 patch(f"{_MOD}.OrchestratorClient", return_value=client), \
                 patch(f"{_UTIL}.provision_git_gate", provision or Mock()):