refactor(backend): move git-gate provisioning to a neutral module

`backend/docker/gateway_provision.py` had no docker-specific code left once
`DockerGatewayTransport` moved out — `provision_git_gate` / `deprovision_git_gate`
drive any `GatewayTransport`, and the guest-side paths they write
(`/git-gate/creds/<id>`, `/git/<id>`, `/etc/git-gate/...`) are identical inside
every backend's gateway. Yet the neutral `backend/consolidated_util.py` reached
into the docker package to import them.

Move it to `backend/gateway_provision.py`, a sibling of its only importer. The
gateway *package* can't host it (git_gate already imports gateway.git_gate,
so gateway importing git_gate would cycle), but the backend layer uses git_gate
freely. Docstrings + the git_gate/service.py pointer updated.

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

bot_bottle/git_gate/service.py

@@ -60,7 +60,7 @@ class GitGate:
         # `sh`, so the script needs the x bit. The gateway copy does not
         # necessarily preserve this mode (`docker cp` does, the Apple `container
         # cp` does not), so provisioning re-applies +x on the gateway side — see
-        # backend/docker/gateway_provision.py.
+        # backend/gateway_provision.py.
         access_hook.chmod(0o700)
         upstreams_with_files: list[GitGateUpstream] = []
         for u in upstreams: