Replace CHANGELOG.markdown with GitHub Releases

[hownowstephen]

Summary

Closes #142

• Deleted stale CHANGELOG.markdown (was missing 5.5.0–6.0.0, had wrong unreleased version)
• Updated gemspec changelog_uri to point to GitHub Releases page
• Removed CHANGELOG.markdown from gem.files
• Added .github/release.yml for auto-categorized release notes (breaking, added, fixed, docs)
• Backfilled GitHub Releases for all previously untagged versions (v0.6.0, v0.6.1, v0.7.0, v1.0.0, v4.3.0, v4.3.1, v4.3.2)

Going forward

Use gh release create vX.Y.Z --generate-notes when cutting releases — GitHub will auto-categorize merged PRs by label using the .github/release.yml config.

Test plan

• Verify gem build customerio.gemspec succeeds without CHANGELOG.markdown
• Confirm changelog_uri on RubyGems points to releases page after next publish

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Note

Low Risk
Low risk: documentation/release-process only changes that affect gem packaging metadata, not runtime code paths.

Overview
Moves changelog management from the repo to GitHub Releases by deleting CHANGELOG.markdown and adding .github/release.yml to auto-categorize generated release notes by PR labels.

Updates customerio.gemspec to stop shipping CHANGELOG.markdown in the gem and to point changelog_uri metadata at the GitHub Releases page instead of the removed file.

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