Fail sandbox CI when the Docker integration prerequisite is unavailable

Severity: high | Confidence: 1 | Effort: S

Where:

• .github/workflows/ci.yml:221-247
• plugins/ca-sandbox/tools/vitest.config.ts:3-18
• plugins/ca-sandbox/tools/run.test.ts:123-128
• plugins/ca-sandbox/tools/lifecycle.test.ts:217-222
• plugins/ca-sandbox/tools/tests/isolation.test.ts:39-44

Evidence: The required sandbox job runs npm test and assumes ubuntu-latest provides a working Docker daemon, but every real-container suite converts a failed docker info probe into describe.skip and the Vitest configuration explicitly keeps no-Docker hosts green. Reproduced by launching the installed Vitest with a child PATH containing Node but not Docker: exit code 0, 15 files passed, 1 file skipped, 164 tests passed, and 31 integration tests skipped. The required CI job has no Docker preflight or executed-integration sentinel, so this same degraded run satisfies merge readiness.

Impact: A runner Docker outage, daemon startup failure, permission regression, or image/runtime incompatibility can silently remove the only real isolation, mount, network, lifecycle, and teardown evidence while the sandbox contract reports green. Pure argv-builder tests cannot prove the untrusted-repository containment guarantees that the skipped suites own.

Recommendation: Give CI a required-integration mode: perform a failing docker info preflight and make Docker probe failures fatal when that mode is set, while retaining local self-skip behavior for developer machines. Emit and assert an integration sentinel or test count so the required job proves at least one real-container suite executed.

Acceptance criteria:

• The ca-sandbox-tools CI job fails before tests when docker info is unavailable or non-zero.
• The same test command remains optionally self-skipping for an explicitly local/non-required mode.
• Required CI records a machine-checkable sentinel proving the real-container isolation, lifecycle, mount, network, and teardown layer executed rather than skipped.
• A test that masks Docker from PATH reproduces a non-zero required-mode result.