[changelog] Add Keep a Changelog automation (#28)

[coisa]

Summary

Added Keep a Changelog automation with the following capabilities:

• Bootstrap: Bootstrapper class to create .keep-a-changelog.ini and CHANGELOG.md when missing
• History Generation: HistoryGenerator to render markdown from git release history
• Validation: UnreleasedEntryChecker to compare unreleased entries against git baseline
• Classification: CommitClassifier to classify commits into keep-a-changelog sections

New Commands

• changelog:init - Bootstraps changelog automation assets
• changelog:check - Validates unreleased changelog entries

Changes

• Added 15 classes in src/Changelog/ namespace
• Added ChangelogInitCommand and ChangelogCheckCommand
• Added reusable GitHub Actions workflows in resources/github-actions/
• Added documentation in docs/usage/changelog-management.rst
• Improved SyncCommand to synchronize changelog assets
• Updated GitHub Actions to use PHP 8.3
• Added comprehensive test coverage for all changelog classes

Testing

• composer dev-tools tests - All 21 changelog tests pass
• composer dev-tools code-style - ECS passes

Changelog

• Added version 1.5.0 with all changelog automation features
• Documented version 1.0.0 with initial plugin capabilities

Closes #28