Builds the application child list for a given runtime role and environment.

The for_role/3 function returns a list of supervisors and workers based on:

• the runtime role ("web" vs everything else)
• the Mix environment (test, dev, prod)
• the configured DAG folder

In test, DAG runtime pieces (pooler, leader, loader, watcher) are skipped. In prod, the file watcher is disabled. In dev, the watcher is enabled to reload DAGs on file changes.