Fix release workflow not extracting annotated tag message

actions/checkout with fetch-tags: true doesn't reliably fetch
annotated tag objects when triggered by a tag push event. Adding
an explicit `git fetch --tags --force` step ensures the annotation
is available before we try to read %(contents:body).

Also switch from %(contents) to %(contents:body) to omit the
redundant subject line ("Release X.Y.Z") from the release notes,
since the tag name already serves as the release title.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>