A kept fork's pull is checked on its combined commit, so its green is the green of what lands #9932

For: the check on a kept fork's pull tests the tree that will land (combined/N), not the author's head alone. docs/features/upstream-landing.md says "Checks run on the pull's head, not on the combined commit, and the card says that too" and lists it under 'Not yet'.

Done looks like:

• [x] When a pull is from a kept fork and combined/N exists, the pull's fast check runs on combined/N's commit. The run records that oid, and the checks line says "checked as combined with main at abc1234".
• [x] checkStale treats that green as current while combined/N is still built on the base tip.
• [x] The card's line "checks run on the head" goes away. The doc's sentence and 'Not yet' item are updated.
• [x] A test with a two-repository fixture, as in test/combined-fork.test.ts, asserts that the queued check's commit is the combined one and that a pull from inside one repository is still checked on its head. It fails on main.

Files: src/checks/ (where a pull's check commit is chosen), src/git/combined.ts, src/core/combined_view.ts, a test, docs/features/upstream-landing.md. Risk: the check pipeline is shared. Keep the non-fork path byte-identical and run test/pull-check*.test.ts before pushing.

Reviewed pull #9940 at its head: ticked When a pull is from a kept fork and combined/N exists, the pull's fast check runs on combined/N's commit. The run records that oid, and the checks line says "checked as combined with main at abc1234".; checkStale treats that green as current while combined/N is still built on the base tip.; The card's line "checks run on the head" goes away. The doc's sentence and 'Not yet' item are updated.; A test with a two-repository fixture, as in test/combined-fork.test.ts, asserts that the queued check's commit is the combined one and that a pull from inside one repository is still checked on its head. It fails on main..