ci: add advisory macOS Apple Container integration runner

Adds an `integration-macos` job that runs the integration suite against
BOT_BOTTLE_BACKEND=macos-container on a self-hosted host-mode macOS runner
(label `macos`), plus the PRD and provisioning docs.

The job is advisory (push-to-main + workflow_dispatch only, never PRs, not
in coverage.needs) since it targets a single non-redundant laptop. It
preflights `container`/`backend status` so a misprovisioned runner fails
loudly, serializes on a concurrency group, and tears down the
`bot-bottle-mac-infra` singleton on exit (#425).

Also relaxes the TestSandboxEscape CI skip guard: it skipped every backend
but firecracker under GITEA_ACTIONS, which would also skip on a host-mode
macOS runner. The guard's real target is the containerized act_runner, so
it now allows both host-mode backends (firecracker, macos-container)
through — otherwise the macOS job would go green while skipping the one
end-to-end test that proves the backend launches.

Closes #426

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

tests/integration/test_sandbox_escape.py

@@ -67,14 +67,25 @@ _DUMMY_HOST_KEY = (
 )
 
 
+# Backends whose CI runner is HOST-mode (self-hosted), so the test process
+# and the backend share a host. The containerized act_runner (docker on
+# ubuntu-latest) is the one that can't see the host bind mount egress_tls_init
+# uses and hides sibling-gateway network topology; host-mode runners
+# (firecracker/KVM, macos-container) don't have those constraints, so the test
+# runs there. Keep this in sync with the `runs-on` labels in
+# .gitea/workflows/test.yml.
+_HOST_MODE_CI_BACKENDS = frozenset({"firecracker", "macos-container"})
+
+
 @skip_unless_selected_backend_available()
 @unittest.skipIf(
     os.environ.get("GITEA_ACTIONS") == "true"
-    and os.environ.get("BOT_BOTTLE_BACKEND") != "firecracker",
-    "skipped under act_runner unless BOT_BOTTLE_BACKEND=firecracker: "
+    and os.environ.get("BOT_BOTTLE_BACKEND") not in _HOST_MODE_CI_BACKENDS,
+    "skipped under the containerized act_runner (docker on ubuntu-latest): "
     "egress_tls_init uses a host bind mount the runner container can't "
     "see, and the network topology hides sibling-gateway visibility — "
-    "these constraints don't apply on the self-hosted KVM runner",
+    "these constraints don't apply on the self-hosted host-mode runners "
+    "(firecracker/KVM, macos-container)",
 )
 class TestSandboxEscape(unittest.TestCase):
     """End-to-end attacks against a real bottle. The bottle stays