feat: initialer Commit des csv2excel CLI-Tools

Go-CLI-Tool zum Konvertieren von CSV-Dateien in Excel (.xlsx),
mit Mage-Build-System und Architektur-Dokumentation.

CLAUDE.md

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+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Project Overview
+
+`csv2excel` is a Go CLI tool that converts one or more CSV files into a single Excel (.xlsx) file, placing each CSV as a separate worksheet.
+
+## Build
+
+This project uses [Mage](https://magefile.org/) (`magefiles/` — eigenes Go-Modul).
+
+```bash
+mage Build         # aktuelles OS (auto)
+mage BuildWindows  # dist/csv2excel.exe
+mage BuildLinux    # dist/csv2excel
+mage Install       # nach $GOBIN
+mage Clean         # dist/ entfernen
+```
+
+Ohne Mage: `go build -o dist/csv2excel.exe .`
+
+## Weiterführende Dokumentation
+
+- **Architektur & Dependencies:** Siehe `docs/architecture.md`