The landing gate itself lands an upstream pull as a merge, so the upstream's commits stay ancestors of main #3558

For: proof that following an upstream does not squash its history (split from #3442).

Done:

• [ ] A test opens an upstream pull through the follow check and lets the landing gate land it (not a direct mergePull), then asserts the upstream tip is an ancestor of main and the next check reads up_to_date.
• [ ] Changing the gate's strategy for an upstream pull to squash makes that test fail.

Files: test/upstream_follow.test.ts, and the landing code only if the test finds it squashes. Risk: the gate in a test is slow; build the fixture once per file and keep the test under 1 s.