Set up JSON schema validations.

Run those:

- at runtime and open up issues when there are violations. a kind of low-tech error reporting system.
- during build or test time.

Start by clarifying how JSON schemas should be defined and which runtime, build, and test paths must validate them. The work is done when violations are reported during runtime and also detected during build or test time, with the reporting behavior agreed and covered.

Written by the indexing model from the issue text.