feat(openshell): add experimental lifecycle adapter

[mogul]

Context

Adds the first bounded OpenShell adapter layer on top of the backend registry.

Changes

• Supports only OpenShell 0.1.4 with a selected local Docker gateway.
• Adds shell-only lifecycle, controlled workspace bind mounts, explicit upload fallback, --clone scratch mount handling, copy, structured output parsing, and host-authoritative state.
• Keeps OpenShell explicit-only and experimental.
• Fails closed for agent kits, providers/secrets, policy compilation, ports, public services, MXC, Kubernetes, and remote gateways.
• Cleans OpenShell and host clone state when readiness fails.

Stack

Base: #581. This PR must merge after the backend registry refactor.

Verification

• PATH="/usr/bin:$PATH" ACQ_BATS_JOBS=1 ./scripts/test-acq-bats (752 passing)
• npm run lint
• ShellCheck batch for wrapper, adapters, stubs, and Bats files
• git diff --check
• Live verification: BLOCKED, because no reviewed local OpenShell gateway is selected on this host.

Rollback

Revert commit HEAD from this PR after the base refactor is merged.

Security Impact

Adds a deliberately constrained experimental execution path. Bind mounts require explicit local-gateway enablement; no providers, credentials, kits, policy expansion, or service exposure is enabled.