docs: update CHANGELOG.md for v0.1.0

CHANGELOG.md

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+# Changelog
+
+All notable changes to this project will be documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
+and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+## [Unreleased]
+
+## [v0.1.0] - 2026-03-05
+
+### Added
+
+- Initial release: CLI tool to convert one or more CSV files into a single Excel (.xlsx) workbook
+- Each CSV file becomes a separate worksheet named after the source filename
+- Auto-detection of CSV delimiter (`,`, `;`, `\t`) based on first-line analysis
+- Support for UTF-8 and Windows-1252 encoding (`-enc` flag)
+- Configurable output file path (`-o` flag)
+- Tolerant CSV parsing with `LazyQuotes` for malformed files
+- Build system via [Mage](https://magefile.org/) with targets for Linux, Windows, and install
+- Version info injected at build time via git tags (`ldflags`)
+
+### Documentation
+
+- README with installation instructions, flag reference, and usage examples