Turn Heroku deploys into a readable Slack changelog

Whenever a new release is deployed to my Heroku app, publish a readable deploy announcement to my Slack channel. Trigger this by polling Heroku for a new release (a deploy) on the app I specify. I will tell you which Heroku app to watch and which GitHub repository holds that app's code.

When a new release fires, use Heroku's Get Release to pull the details of the new version: the version number, the description, who deployed it (the deployer), and the timestamp it went live. Also determine the timestamp of the previous release for this app so you know the window of changes to summarize.

Use GitHub's List Pull Requests on the mapped repository to gather the pull requests that were merged since the previous release. Filter to closed pull requests and keep only the ones that were actually merged and whose merge time is after the prior release's timestamp. Ignore pull requests that were closed without merging, and ignore anything merged before the last deploy.

From those merged pull requests, draft a plain-English "What shipped in vN" changelog using the new version number. Summarize what actually changed in clear language, group related changes where it helps readability, and keep it skimmable. Lead with the version number, name the person who deployed, and note the time it went live.

Check the release description before writing. If it reads "Rollback to vX", clearly flag the announcement as a rollback to that earlier version instead of listing new changes, and say who performed the rollback. If there are no merged pull requests since the previous release, still post a short note confirming the deploy rather than an empty changelog.

Post the finished summary to my Slack channel using Slack's Send a Message. Format it with Slack's message formatting (single asterisks for bold, and <url|text> for links) so it reads cleanly in the channel.