[sabtain05]

Problem

The mods registry is edited regularly, but the repository had no executable validation. The documentation tells contributors to validate entries against the schema, while the latest registry history includes a follow-up formatting fix after an entry update. A malformed or duplicate entry could therefore reach the live mods hub.

What changed

• Added a dependency-free Node validator for registry shape, field constraints, and unique mod names.
• Added regression tests for the checked-in registry, duplicate names, and overlong descriptions.
• Added a GitHub Actions workflow that runs the validator and tests on registry changes.
• Documented the automated check in mods/README.md.

Validation

• node mods/validate_registry.mjs
• node --test mods/validate_registry.test.mjs
• node --check mods/validate_registry.mjs
• node --check mods/validate_registry.test.mjs
• git diff --check

This PR does not change the registry contents or the mods hub behavior; it prevents invalid registry changes from merging.