Release v1.5.1

Fixed

• A hotfix waiting alongside a pending major release now fails only the run started by merging the hotfix itself. Scheduled ticks and other merge runs warn and keep the batch pending. (#218)
• Batched release notes now combine the summaries of every pull request into one leading ## Summary, instead of repeating a summary per pull request. (#218)
• Batched releases now warn once for each merged pull request that has no release-intent label. (#218)
• Markdown reference-link definitions in a pull request description are kept when release notes are aggregated. When two pull requests define the same label, the first definition is kept and a warning names the duplicate. (#218)