Set up changelog-driven release workflow

[apermo]

Automated release workflow matching the apermo-surf pattern.

Triggered on push to `main`, reads version from `CHANGELOG.md`.

• Extract version from first `## [x.y.z]` heading in CHANGELOG.md
• Skip if tag already exists
• Create git tag
• Create GitHub release with changelog notes
• Comment on merged release PR
• Trigger wordpress.org deployment (see Automated deployment to wordpress.org SVN #27)
• Trigger Packagist update (see Register on Packagist for Composer installation #28)