feat(#22): md_to_text before filter by language

[h1alexbel]

closes #22

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PR-Codex overview

This PR updates the code to read the CSV file name from an environment variable, applies a new filtering step to convert readme content to text, and improves CSV export functionality.

Detailed summary

• Updated to read the CSV file name from an environment variable
• Added a step to convert readme content to text during filtering
• Improved CSV export by excluding the index column

The following files were skipped due to too many changes: tests/filtered-with-null-topics.csv, tests/filter-expected.csv

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[h1alexbel]

@rultor merge

[rultor]

@rultor merge

@h1alexbel OK, I'll try to merge now. You can check the progress of the merge here

[rultor]

@rultor merge

@h1alexbel Done! FYI, the full log is here (took me 7min)