Forgejo sets a label and will notify this when opening the pull
request. Triggering when it opens will make two workflows for the same
SHA. Re-opening is a border case that is not needed.
* if <!-- is inserted just after a <!-- --> it will not render
well, it needs to be separated by a newline
* do not use ? in sed -E, it is not the same as with JavaScript
GITHUB_TOKEN does not have permission to write the repository and is
not allowed to edit or comment on pull requests because of that. A PAT
from a regular user who does **not** have permission to write to the
repository either but who is in a the contributors team will have
permissions to do that because there is a "write pull request"
permission given to the team.
If the 'worth a release-note' label is set, add a release note entry
to the description of the pull request as a preview.
* use the `release-notes/<pr-number>.md` file if any
* otherwise use the pull request title
Refs: https://code.forgejo.org/forgejo/release-notes-assistant
