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/gateway_provision.py

@@ -15,10 +15,10 @@ bottle's push credentials out of another's repos on the shared gateway.
 from __future__ import annotations
 
 import re
-from typing import Protocol
 
 from .util import run_docker
 from ...git_gate import GitGatePlan, git_gate_render_provision
+from ...gateway import GatewayProvisionError, GatewayTransport
 
 # bottle ids index the gateway's per-bottle repo + creds dirs; they land in
 # exec/cp path arguments, so validate before any path is built (a traversal
@@ -27,25 +27,6 @@ from ...git_gate import GitGatePlan, git_gate_render_provision
 _SAFE_BOTTLE_ID = re.compile(r"[A-Za-z0-9_-]+")
 
 
-class GatewayProvisionError(RuntimeError):
-    """A git-gate provisioning step against the running gateway failed."""
-
-
-class GatewayTransport(Protocol):
-    """How the launcher stages files + runs commands in the running gateway.
-    Backend-neutral so the same provisioning logic serves the docker gateway
-    (exec/cp over the docker socket) and the firecracker gateway VM (over
-    SSH)."""
-
-    def exec(self, argv: list[str]) -> None:
-        """Run `argv` in the gateway, raising `GatewayProvisionError` on
-        failure."""
-
-    def cp_into(self, src: str, dest: str) -> None:
-        """Copy host file `src` to `dest` in the gateway, raising on
-        failure."""
-
-
 class DockerGatewayTransport:
     """`GatewayTransport` for the docker gateway container (exec/cp)."""