Create an Exams.fit bulk tools import ZIP from the supplied template.

Requirements:
1. Every item type is tool, every slug starts tools/, and every category type is tool_category.
2. Use only supported data.tool_type values: grade_calculator, percentage_calculator, unit_converter, date_countdown, or study_timer.
3. Configure inputs in data.fields. Do not include executable PHP, JavaScript, formulas for eval(), remote scripts, iframes, or tracking code. Exams.fit supplies the safe tool engine.
4. Write at least 600 useful visible characters explaining the calculation, a worked example, assumptions, interpretation, privacy, and limitations.
5. Use unique EF-USA-###### IDs and exact SHA-256 item checksums in manifest.json.
6. Set status draft and noindex true. The administrator reviews and publishes tools.
7. Return one valid ZIP with manifest.json at root and JSON items under items/. Follow docs/TOOLS-BULK-IMPORT.md.
