docs: add CHANGELOG.md for v1.1.0 release

Add comprehensive changelog documenting the initial PyPI release
with all features and infrastructure improvements.

Author: najibninaba

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/1.0.0/),
+and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+## [1.1.0] - 2025-08-14
+
+### Added
+- Initial release of bmad-pack-installer to PyPI
+- Command-line interface for installing BMAD expansion packs
+- Package validation with hash verification
+- Symlink management for pack installations
+- YAML manifest support for pack definitions
+- Comprehensive test suite with pytest
+- Type hints and mypy support
+
+### Features
+- Install packs from local or remote sources
+- Validate pack integrity using SHA256 checksums
+- Manage symlinks for pack directories
+- Rich CLI output with progress indicators
+- Support for Python 3.9+
+
+### Infrastructure
+- Restructured package layout for PyPI distribution
+- Automated testing with pytest
+- Code formatting with Black
+- Type checking with mypy
+- Build system using hatchling