docs(adr): define OpenShell backend contract

Outcome

Adopt an ADR that freezes the OpenShell backend security and compatibility contract before adapter code lands.

Define the supported OpenShell CLI floor, gateway ownership model, and initial experimental support state.
Define initial compute-driver status: local drivers are candidates subject to live verification; MXC and Kubernetes require separate acceptance evidence before support is claimed.
Preserve neutral --image / ACQ_IMAGE, kit-installed agent/provider responsibilities, host-authoritative state, and explicit backend selection.
Define approved host-workspace bind-mount behavior, --clone scratch-clone mounts, and upload/download fallback semantics.
Define provider/profile ownership, default-deny policy compilation, agent-ancestry process boundaries, and credential endpoint constraints.
Record lifecycle, port, copy, kit, provider, and cleanup capability expectations.

Run Markdown validation and link checks. Obtain architecture/security review before implementation claims rely on the decision.