Add GitHub Actions workflow to schedule and auto-merge blog PRs

[elichad]

Can work for any PR, but we'd use it for blog posts.

Currently we merge blog posts ahead of their intended publication date, and rebuild the site daily to publish the posts. However, the RSS feed gets updated with the post immediately after merging - this is inconvenient for anyone who subscribes to the feed as they will get a notification but the post link won't yet work. In particular, I would like to automatically cross-post the RSS feed to Slack (and in future other platforms) but the RSS Slack app doesn't support scheduling the message into the future, it just sends right when the feed is updated.

Changing the publication date before merging is a bit of a faff. Remembering to merge on the correct date is also impractical. So this workflow handles scheduling a merge on the correct date. All we need to do is add the following syntax at the end of the description in PRs that we want to schedule:

/schedule 2026-01-15

[elichad]

After a bit more investigation:

• this action doesn't work on forks, so it isn't great for our use case
• the RSS issue can be fixed at source by creating the feed ourselves (it's basically due to a bug in jekyll-feed, but jekyll-feed has not had updates since two years ago, so I'm not sure it will get fixed)

I'm going to close this and take that other route.