refactor(acq): generalize backend registry and capabilities #575

Outcome

Replace SBX/MSB-only enumeration with a registry and core-enforced capability contract that admits OpenShell without changing current backend behavior.

Scope

• Centralize backend metadata used by resolution, help, backend list, doctor, image guidance, secret propagation, and tests.
• Preserve resolution order and current MSB/SBX auto-detection behavior.
• Register OpenShell as explicit-selection-only; do not change automatic selection.
• Operationalize lifecycle, mount, clone, copy, ports, provider binding, rotation, kit-reapply, host-config, network translation, and image-selection capabilities.
• Keep SBX and MSB public command shapes and semantics compatible.

Verification

Add/extend offline Bats coverage for selection, registry output, unavailable backends, capability gating, and unchanged SBX/MSB routing. Run the full offline suite and standard lint/security checks.