fix: gate Unix-only OCI test helper

[ORESoftware]

Follow-up to #266. The semantic integration was merged before its final Windows all-target Clippy run completed. That run exposed one Unix-only OCI test helper that was still compiled on Windows and therefore reported as dead code.\n\nThis exact one-line refinement adds #[cfg(unix)] to the helper whose callers and shell fixture are already Unix-only. It does not change production behavior.\n\nValidation before publication:\n- cargo fmt --check\n- cargo clippy --locked --all-targets -- -D warnings\n- full #266 integration suite: 502 tests passed; two explicit Nix closure tests ignored\n\nLinear: DEN-3495

[ORESoftware]

Current main already contains this exact one-line Unix-only helper gate as commit 5adf09b. git cherry main origin/agent/binary-zip-publish marks 1f6e8de patch-equivalent, so closing avoids manufacturing a duplicate merge while preserving all behavior and history.