feat: add macos container backend scaffold

bot_bottle/backend/macos_container/launch.py

@@ -0,0 +1,34 @@
+"""Launch flow for the macOS Apple Container backend.
+
+The backend is registered and its host primitives are implemented, but
+full launch is intentionally blocked until the sidecar network
+enforcement design is finished. Apple Container can publish ports and
+create networks, but bot-bottle's Docker topology relies on an agent
+container attached only to an internal network while the sidecar bundle
+also has egress. The first runnable version must preserve that
+no-direct-egress property.
+"""
+
+from __future__ import annotations
+
+from contextlib import contextmanager
+from typing import Callable, Generator
+
+from ...log import die
+from .bottle import MacosContainerBottle
+from .bottle_plan import MacosContainerBottlePlan
+
+
+@contextmanager
+def launch(
+    plan: MacosContainerBottlePlan,
+    *,
+    provision: Callable[[MacosContainerBottlePlan, "MacosContainerBottle"], str | None],
+) -> Generator[MacosContainerBottle, None, None]:
+    del provision
+    die(
+        "macos-container backend launch is not enabled yet: "
+        "the backend primitives are present, but sidecar network "
+        "enforcement still needs implementation."
+    )
+    yield  # pragma: no cover