Automerge PRs action

[yarikoptic]

xref:

• https://github.com/bids-standard/bids-specification/issues/147

Inspired by

• https://github.com/bids-standard/bids-specification/pull/1774#issuecomment-2115310934

TODOs

• [x] create automerge label (created, we didn't have any)
• [ ] fix that crap chatgpt produced... or just find a working example of such workflow elsewhere...
• [ ] see if all desired outputs actually produce what is needed
• [ ] may be test on some other repo where we can trick the dates of commits etc
• [ ] uncomment and do provide GITHUB_TOKEN