Change release CI flow to actually use our CHANGELOG.md notes

[JesseHerrick]

Our previous release flow was having GitHub's AI automatically generate the changelogs in the official release. These differed somewhat from our actual CHANGELOG.md notes. Because many automations rely on these Release notes as the main changelog, this change ensures that they're always in sync. Our current flow is to always push a changelog update before tagging and releasing, so this flow should work well.

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Note

Low Risk
Only affects release automation and maintainer docs; releases now fail fast if the changelog section is missing, which is intentional guardrail behavior.

Overview
GitHub releases now publish the same text as CHANGELOG.md instead of auto-generated notes.

The release workflow adds an Extract release notes step that pulls the ## [version] section for the tag (stripping the v prefix) into release-notes.md, and fails the job if that section is empty. gh release create then uses --notes-file release-notes.md instead of --generate-notes.

The Releasing section in the README tells maintainers to review the matching changelog entry before tagging, since that section becomes the official release notes.

^{Reviewed by Cursor Bugbot for commit 3703fb5. Bugbot is set up for automated code reviews on this repo. Configure here.}