chore(opapi): Add CHANGELOG.md

[sea-grass]

This PR adds a changelog for historical versions of opapi. This will help in automated upgrades between opapi versions. For added/changed features, usage examples have been included along the changelog notes.

(vibed)

[greptile-apps]

Greptile Summary

This PR adds a changelog for @bpinternal/opapi. The main changes are:

• New opapi/CHANGELOG.md with historical release notes.
• Usage examples for notable added and changed features.
• An unreleased section for current package-file spelling fixes.

Confidence Score: 5/5

This looks safe to merge.

• No blocking issues found in the changed documentation.

Important Files Changed

Filename	Overview
opapi/CHANGELOG.md	Adds historical changelog entries and examples for opapi releases.

_{Reviews (1): Last reviewed commit: "chore(opapi): Add CHANGELOG.md" | Re-trigger Greptile}