Add option to lint toc before proceeding

This is a lot more convenient than manually fixing it: I did it with a little `regex` trick here: https://github.com/danburzo/percollate/pull/63#issuecomment-4444628536

The issue names no files or tests; start by locating the CLI option handling and the point where TOC generation proceeds. Review the linked example to clarify the intended lint behavior, then define completion as an option that checks the TOC before processing continues.

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