A trusted source's big first fold opens one pull, not a flood, and lands by true merge after the crew reviews it #9024

From #9003: the owner follows four trusted sources (jmoran/forge-jay, tdobbertin-espo/forge, benjivers/forge, and foundry from forge-apps.benjivers.com). The first fold from jmoran is 149 commits with about 90 conflicting files against main.

Done when:

• [ ] The first sync of a followed source that is many commits ahead opens or updates exactly one pull stamped with its source. It does not open one per commit or one per poll, and a test with a fake Forgejo 100+ commits ahead proves it.
• [ ] A first fold that conflicts goes to crew review and resolution (the slices from #9010), even when the source is trusted. It does not take the no-review path from #9015, and a test covers that branch.
• [ ] It lands as a true merge of the source's head, so the author's commits are ancestors of main and git merge main on their side is clean or a fast-forward.
• [ ] Later commits on the same source update that one pull while it is open.

Files: src/forks/ (the upstream follow), src/pulls/ merge path, test/. Risks: a poll racing the first sync opens two pulls; key the pull by source and branch.

From #9003: once this lands, the owner will follow the four sources live. Say so on the pull, so they know when it is out.