Ignore user-managed taxonomy workbook and generated artifacts; remove committed Excel workbook

[Spbd1]

Motivation

• The real taxonomy workbook is a user-managed external file and should not be stored in the repo to avoid leaking a large or sensitive file and to keep the repository lightweight.
• Generated Excel exports and reports under data/ are local artifacts that should not be committed.
• The README should clearly explain how to import the real workbook from the dashboard or CLI instead of shipping it in the repository.

Description

• Added ignore rules to .gitignore for data/taxonomy/imports/*.xlsx, data/taxonomy/exports/*.xlsx, data/reports/*.xlsx, data/reports/*.html, data/reports/*.pdf, and global *.xls/*.xlsx so local workbooks and exports/reports are not committed.
• Removed the committed Excel workbook argument_risk_taxonomy_living_workbook_v2_taxonomy_first.xlsx from the repository to make the canonical taxonomy a local/imported artifact.
• Updated README.md to emphasize that the real taxonomy workbook is user-managed, provide a short Taxonomy workbook imports section with an example CLI command (python scripts/import_taxonomy_excel.py --input data/taxonomy/imports/argument_risk_taxonomy_living_workbook_v2_taxonomy_first.xlsx), and note that generated exports/reports are ignored and empty import/export/report directories are retained via .gitkeep.

Testing

• No code logic was modified, only repository metadata and documentation were changed, so no automated tests were required or modified.
• No automated tests were run as part of this change.

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Codex Task