Show untagged rows and link to create the next tag (schema 5)

[lesnik512]

Shows rows that no tag ships yet and links to GitLab's new-tag form for the head commit.

Each service gets an optional untagged field in the report: rows, head_sha, next_tag and create_url.

• rows counts the rows above the newest tagged row, or every row when none has a tag.
• next_tag is the highest X.Y.Z tag (with an optional v prefix) with its minor version bumped. Versions compare numerically and pre-release tags are ignored. It is null when no tag has that form.
• create_url is <project>/-/tags/new?tag_name=<next_tag>&ref=<head_sha>, or only ref without a next_tag. GitLab's projects/tags/new.html.haml reads both params to fill in the name and the "Create from" ref.
• The field is null when the head is tagged or the service has no rows. --jira reports get it as well.

On the page, the service line shows "N untagged", and the opened service shows a "Create tag X" link. This replaces the old "no tag yet" hint. The copyable release post is unchanged.

When the walk stops at RELEASE_SCOPE_MAX_COMMITS before reaching a tag, the count is a lower bound. The existing "range cut short" badge sits next to it.

The report schema goes to 5, so the skill and README pin release-scope>=0.7,<0.8, as the schema 4 change did. The skill also gains the --exclude flag from 0.6.2.