release_next.sh drops pin comments in the snapshot and has no guard for a bare --release-tag #734

Two small rough edges in hack/release_next.sh, both there before #710.

The snapshot step pins cozystack_version and cozystack_tag with a sed that replaces everything after the key. That also drops the trailing comments next.yaml carries on those lines (# bare, as used by helm --version and # v-prefixed, as used in GitHub URLs). You can see it in data/versions/v1.6.yaml, where both lines have no comment. Matching only the quoted value, for example s|^\(cozystack_tag:[[:space:]]*\)"[^"]*"|\1"${RELEASE_TAG}"|, keeps the comment.

--release-tag without a value reads $2 under set -u and dies with "unbound variable" instead of the usage error. hack/update_versions.sh checks $# -lt 2 for its value-taking flags, and the same guard fits here.