JSON output format for agent and script consumption

Fabrik now supports a machine-readable output option. Pass --format json to commands that emit structured data: cache stats become a single JSON object, targets are printed as NDJSON with their computed labels, and the run command returns a JSON outcome record with the label, target kind, action digest, cache state, exit code, and output path. This makes it easier for scripts and agents to consume Fabrik’s output without parsing prose.