docs(adr): define OpenShell backend contract

[mogul]

Context

Adds the architecture and security contract required before OpenShell adapter implementation.

Changes

• Defines OpenShell as explicitly selected and experimental.
• Establishes per-driver promotion gates for local runtimes, MXC, and Kubernetes.
• Preserves host bind mounts, --clone, neutral image selection, kit/provider role boundaries, host-authoritative state, and acq secret authority.
• Requires default-deny policy enforcement across agent process ancestry and real live verification evidence.

Verification

• PATH="/usr/bin:$PATH" ACQ_BATS_JOBS=1 ./scripts/test-acq-bats test/bats/20-backend-resolution.bats test/bats/60-help.bats
• npm run lint
• git diff --check

Rollback

Revert commit 9cb0553 to remove the proposed ADR.

Security Impact

Defines, but does not yet implement, security requirements for gateway ownership, credential handling, host mounts, egress, and driver promotion.