Fetch annotated tag objects in release workflow

actions/checkout@v4 does not fetch annotated tag objects by default even
with fetch-depth: 0. Without the tag object present locally, git tag -l
--format='%(contents)' falls back to the commit message rather than the
tag annotation. Adding fetch-tags: true ensures the annotated tag object
is available when the release notes are extracted.

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