refactor(git-gate): make GitGate a service class in a git_gate package

Turn the git_gate module into a package with GitGate as a concrete service class
(dropping the ABC), mirroring the Supervisor shape. The host-side git-gate
operations are now methods the backend drives:

  git_gate/
    __init__.py  — thin __getattr__ facade (keeps `from bot_bottle.git_gate
                   import …` working; render/hook names lazily forwarded to
                   gateway.git_gate_render)
    plan.py      — GitGatePlan (the launch DTO the backend contract references)
    service.py   — GitGate: prepare / provision_dynamic_keys /
                   revoke_provisioned_keys / preflight_host_keys
    provision.py — deploy-key lifecycle (was git_gate_provision.py)
    host_key.py  — host-key preflight (was git_gate_host_key.py)

The backend launch + base.py preflight now call GitGate() methods instead of the
free functions. The runtime rendering / in-gateway hook execution stay in
gateway/git_gate_render.py (data plane) — only the host-side service moved.

Because the facade forwards names lazily, the ~25 `from bot_bottle.git_gate
import GitGatePlan` call-sites are unchanged, and importing git_gate.plan (the
contract's dependency) no longer drags in the provisioning / forge-API code.

Full unit suite green (2243).

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

bot_bottle/backend/macos_container/launch.py

@@ -48,10 +48,7 @@ from ...egress import (
     egress_agent_env_entries,
     egress_resolve_token_values,
 )
-from ...git_gate import (
-    provision_git_gate_dynamic_keys,
-    revoke_git_gate_provisioned_keys,
-)
+from ...git_gate import GitGate
 from ...gateway.git_http_backend import DEFAULT_PORT as _GIT_HTTP_PORT
 from ...image_cache import check_stale
 from ...log import die, info, warn
@@ -151,7 +148,7 @@ def launch(
         except BaseException as exc:  # noqa: W0718 - teardown must continue
             teardown_exc = exc
             warn(f"macos-container teardown failed: {exc!r}")
-        revoke_git_gate_provisioned_keys(bottle_for_revoke, git_gate_dir_for_revoke)
+        GitGate().revoke_provisioned_keys(bottle_for_revoke, git_gate_dir_for_revoke)
         if teardown_exc is not None:
             raise teardown_exc
 
@@ -282,7 +279,7 @@ def _provision_git_gate_keys(
 ) -> MacosContainerBottlePlan:
     if not plan.git_gate_plan.upstreams:
         return plan
-    git_gate_plan = provision_git_gate_dynamic_keys(
+    git_gate_plan = GitGate().provision_dynamic_keys(
         plan.manifest.bottle,
         plan.git_gate_plan,
         git_gate_state_dir(plan.slug),