Add GraphQL introspection detection (#95)

[jthack]

Summary

• parse GraphQL responses for data.__schema / data.__type to spot enabled introspection
• provide fallback regex detection when JSON parsing fails
• register the passive check and enable it in the Balanced preset

Testing

• pnpm lint
• pnpm typecheck
• pnpm test -- --match 'GraphQL introspection'

Closes #95