docs: add CHANGELOG.md file

[HemaOnGit]

Initialize a changelog file to track features, fixes, and updates for the agents framework, following the Keep a Changelog standard.

---

Note

Low Risk
Documentation-only change introducing a new changelog file; no runtime or behavioral impact.

Overview
Adds a new CHANGELOG.md following the Keep a Changelog format, with an initial [Unreleased] section documenting high-level Added and Fixed entries for the project.

^{Written by Cursor Bugbot for commit aff0e19. This will update automatically on new commits. Configure here.}