Add $schema validator

it needs a --schema or --validate, alternatively a -r, -R, -v or -V, so we can take a $schema, schema,json how however it's typically done.

The issue names no files, tests, or entry points. First clarify whether the feature should expose --schema, --validate, or one of the proposed short options, and define the expected schema input and validation behavior. Done should mean the selected interface validates JSON against a $schema and has coverage for the agreed cases.

Written by the indexing model from the issue text.