Change pa.sh to be a python script for better code maintenance

`pa.sh` doesn't need to be a shell script anymore, since it runs the preprocessed script using `runner.sh`. Anything that needs to happen in a script can be done in runner, and then pa.sh can become a better engineered solid program which would improve code maintenance.

Start by reading pa.sh and runner.sh to understand how pa.sh invokes the preprocessed script and which behavior is delegated to runner.sh. Rewrite pa.sh as a Python script while preserving that invocation flow, then verify that the existing PaSh command behavior remains unchanged.

Written by the indexing model from the issue text.