docs: Add initial CHANGELOG with project release history and key updates

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]
+
+### Fixed
+- Fixed a bug where the `scan --no-index` command would not respect the `max_results` config setting (#1)
+
+### Added
+- Integration tests covering indexing and scanning pipelines (#3, #4, #5, #8)
+
+## [0.1.0] - 2025-06-25
+
+### Added
+- Initial alpha release of **Nyx** CLI tool
+- Multi-language AST pattern scanning via `tree-sitter` for Rust, C/C++, Java, Go, PHP, Python, Ruby, TypeScript, JavaScript
+- `scan` command: filesystem walker, pattern execution, console output
+- `index` command: build, rebuild, and status reporting of SQLite-backed index
+- `list` command: list indexed projects with optional verbosity
+- `clean` command: remove one or all project indexes
+- Configuration system with `nyx.conf` (generated) and `nyx.local` (user overrides)
+- Default severity levels: High, Medium, Low
+- Unit tests for core modules (config, ext, project utils)