Grafana JSON Export to Jsonnet

Convert Grafana JSON exports (UI: Share → Export) to Jsonnet using grafana-code unified libraries. Produce a single self-contained file, modernize legacy panels, and preserve row structure.

Not suitable for: Refactoring existing Jsonnet (use grafana-jsonnet-refactor), content optimization (use grafana-dashboard-optimize), or Python report migration (use grafana-report-to-dashboard).