feat(macos): run containers inside a bottle via guest-local podman

Adds the `nested_containers` bottle flag. On the macOS backend it starts
a rootless podman service inside the bottle and exposes its
Docker-compatible API socket, so the agent still runs `docker` and
`docker compose`. No host daemon socket is mounted and the guest gains
no capabilities; backends that cannot run a guest-local engine reject
the flag in the shared prepare template rather than ignoring it.

Podman rather than rootless Docker because Apple Container's capability
bounding set omits CAP_SYS_ADMIN, which the kernel requires to write a
multi-range uid_map via newuidmap. With no subordinate UID range podman
falls back to a single-UID self-mapping an unprivileged process may
write itself, so the image build strips /etc/subuid and /etc/subgid
entries rather than adding them.

That mapping is also why nested containers are not an isolation layer:
root inside one is the agent user outside it. They are a build/test
convenience; the bottle remains the boundary.

Ports the spike branch onto main, renaming docker_access — it implied
Docker and granted access to nothing on the host — and drops podman
from the derived layer now that every built-in image ships it (#451).

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

tests/unit/test_macos_container_launch_wiring.py

@@ -49,6 +49,7 @@ def _plan(
     agent_git_gate_url: str = "",
     agent_supervise_url: str = "",
     image_policy: str = "fresh",
+    nested_containers: bool = False,
 ) -> MacosContainerBottlePlan:
     routes_path = stage_dir / "routes.yaml"
     routes_path.write_text("routes: []\n", encoding="utf-8")
@@ -67,6 +68,7 @@ def _plan(
         manifest=_MANIFEST,
         stage_dir=stage_dir,
         slug="dev-abc",
+        nested_containers=nested_containers,
         container_name="bot-bottle-dev-abc",
         image="bot-bottle-agent:latest",
         dockerfile_path="/repo/Dockerfile",