Add CHANGELOG.md with automated generation

[needsbuilder]

Summary

Add a CHANGELOG.md to track notable changes per release. Optionally set up automated changelog generation from Conventional Commits.

Suggested Scope

1. Create initial CHANGELOG.md with recent release highlights
2. Evaluate tools like conventional-changelog or changelogithub
3. Optionally integrate with scripts/release.mjs to auto-generate on release

Why This Is a Good First Issue

• Documentation-focused, no runtime code changes
• Good way to learn the project's release process
• Conventional Commits are already used in this repo