refactor(acq): register experimental OpenShell backend

[mogul]

Context

Prepares acq to add OpenShell without changing SBX/MSB resolution behavior.

Changes

• Adds a Bash 3.2-compatible backend registry.
• Registers OpenShell as explicit-only until its adapter exists.
• Keeps OpenShell out of auto-detection and preserves MSB/SBX selection behavior.
• Derives backend help, list, doctor, version parsing, and cross-backend propagation iteration from registry metadata.
• Adds regression coverage for explicit-only behavior and updated presentation.

Verification

• PATH="/usr/bin:$PATH" ACQ_BATS_JOBS=1 ./scripts/test-acq-bats (742 passing)
• npm run lint
• git diff --check

Rollback

Revert commit f45a734 to restore the prior SBX/MSB-only enumeration.

Security Impact

No new executable backend path is enabled. Explicit OpenShell selection fails closed until a future adapter is added.