Add release checklist and changelog-driven release notes

[blackopsrepl]

Closes #14.

Summary

• add a repository-level RELEASE.md checklist for public releases
• make CHANGELOG.md maintainable with an Unreleased section and real release entries
• update the release workflow to extract notes from CHANGELOG.md and fail if the version entry is missing
• point maintainers at the checklist from the README release section

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Keep the release job compatible with make bump-*

This hard failure breaks the repository's existing bump-and-tag flow. The documented bump targets in Makefile:165-177 still call commit-and-tag-version, and the repo's own generated release commit (86683a6) shows that helper can create a ## <version> heading with no body at all under this commit history. In that case awk returns an empty string here, so the tag push passes tests but the GitHub Release job aborts. Either the bump helper/config needs to stop auto-generating the changelog/tag, or this step needs a fallback instead of exit 1.

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