Add CHANGELOG and document release process

[cursor]

Summary

Introduce a maintainer-facing CHANGELOG and a short release process so PyPI publishes are repeatable.

Goals

• Add CHANGELOG.md (Keep a Changelog style is fine)
• Document version bump + tag + GitHub Release / workflow steps in CONTRIBUTING or docs
• Record breaking changes from the nested CLI refactor (Refactor CLI to nested command groups #90) in the next release notes

Origin

Carried forward from roadmap #80 “Later ideas”.