Returns a named list of command definitions. Each entry has:
- title
Display name
- description
One-line summary
- args
Named list of argument specs (name, type, required, description, example)
- fn
Function to call with parsed arguments
Returns a named list of command definitions. Each command has a title, description, argument specs (name, type, required, help), usage examples, and an executor function that calls the underlying PRA tool wrapper.