Everything a merge sets off runs with the merger's rights or as the forge: a test merges as a plain writer and finds no write made as another account #5837

Sentinel found a merge follow-up in #5761 acting as the owner on a writer's merge. This makes sure that cannot happen in any follow-up.

Done when:

• [ ] List every follow-up mergePull (src/actions/pulls.ts) starts, in the pull description: issue closes, releases, events, comments, settings and anything else.
• [ ] Each one writes either as the merger, checked against the merger's rights, or as the forge's own system actor, labelled as such. None writes as the pull's author, the repository's owner or autonomyBy.
• [ ] A test merges as a writer who is not an admin, then reads every row the merge wrote (comments, events, settings). It finds no actor besides the merger and the forge's system actor.
• [ ] Any follow-up this finds acting as someone else is fixed in the same pull, with its own test.