Drop CHANGELOG.md in favor of GitHub release notes

[Fivell]

Summary

Removes CHANGELOG.md from the repo. From v6.0.0 onward, release notes live on the GitHub Releases page instead.

Why

• The changelog has historically gone unmaintained for long stretches — the v5.1.0 entry was missing entirely until bump v6.0.0 #217 backfilled it
• GitHub Releases are easier to keep accurate (auto-generated draft from merged PRs, edited at tag time, never out of sync with the tag)
• Releases are linkable from rubygems.org and discoverable from the repo header without an extra file to maintain
• One source of truth — no risk of CHANGELOG.md and the GitHub Release saying different things

Follow-up

When tagging v6.0.0, paste the content from the merged-then-deleted CHANGELOG.md (d79ef68:CHANGELOG.md) as the GitHub Release body. From v6.0.1 onward, draft release notes can be auto-generated from merged PRs.

Test plan

• Nothing in the codebase references CHANGELOG.md (grep -ri changelog returns nothing)
• The gemspec uses git ls-files so the deleted file just stops shipping with the gem