Support to parallel process multiple PDFs from CLI using a rule.json

Possible command: `$ excalibur distribute --rule rule.json directory/`

Start at the CLI entry point for the proposed `excalibur distribute --rule rule.json directory/` command and determine how rules and PDF inputs are currently handled. Clarify the rule.json format, parallel-processing behavior, and expected output before implementation; done means the command processes multiple PDFs from a directory according to the rule and has coverage for the agreed behavior.

Written by the indexing model from the issue text.