Optional object defining custom build inputs for cache invalidation.
This value is hashed during config normalization and used to detect
when builds need to be regenerated due to non-source changes.
Use cases:
- Environment variables baked into build: {api_url: process.env.PUBLIC_API_URL}
- External data files: {data: fs.readFileSync('data.json', 'utf-8')}
- Build feature flags: {enable_analytics: true}
Can be a static object or an async function that returns an object.
IMPORTANT: It's safe to include secrets here because they are hashed and deleted
during config normalization. The raw value is never logged or persisted.