Specify single input and output file

The CLI is cumbersome.

It requires placing the schema file in a directory with a unique extension and then outputting to a directory.

I just want to take a single file and convert it to a single file.

`--input` doc says it takes single file, but actually I get an error that it's not a directory.

Start by tracing the CLI handling for `--input` and how it currently validates directories. Reproduce the documented single-file invocation, then verify that one schema file can be converted to one output file without requiring directory-based input or output.

Written by the indexing model from the issue text.