The lookup was matching on head.ref alone, so running create with a
different --base for the same branch would silently return an
unrelated PR's URL instead of opening a new one. Resolve base before
the check and filter on both head.ref and base.ref.
gitea-pr create now checks for an existing open PR on the current
branch first and returns its URL instead of creating a new one. Only
matches open PRs by design -- once merged, new commits on the same
branch correctly get a new PR.