chore: remove CHANGELOG.md in favor of GitHub Releases (#31)

The GitHub Releases page is the single source of truth for release
notes; the checked-in CHANGELOG.md was redundant and drifted from it.
Nothing in the repo referenced the file.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: anujhydrabadi