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>

docs/ci.md

@@ -2,11 +2,22 @@
 
 The test workflow lives at [`.gitea/workflows/test.yml`](../.gitea/workflows/test.yml).
 It runs the unit suite plus one integration job per backend
-(`integration-docker`, `integration-firecracker`) on:
+(`integration-docker`, `integration-firecracker`, `integration-macos`) on:
 
 - every push to a branch with an open pull request, and
 - every push to `main`.
 
+`integration-macos` is the exception: it is **advisory**, running only on
+push-to-`main` and `workflow_dispatch`, never on pull requests. It targets the
+Apple Container backend on a self-hosted macOS runner (label `macos`,
+registered in host mode — Apple Container can't run in a Linux container, so it
+can't reuse the `kvm` runner). A single non-redundant laptop must not be able
+to block a PR merge, so the job stays out of the `coverage` job's `needs` and
+its coverage never feeds the diff-coverage gate. Because the infra container is
+a singleton (`bot-bottle-mac-infra`), the job declares a `concurrency` group
+and tears the container down on exit; keep runner concurrency at 1. See the
+README "macOS Apple Container" CI note for runner provisioning.
+
 Each integration job selects its backend via `BOT_BOTTLE_BACKEND` and
 runs a **preflight** (`./cli.py backend status --backend=<name>`) that
 prints a clear per-check readiness summary and fails the job when the