warm-caches.yaml: build the dependencies and packages on every platform after each push to trunk and save the stack caches under trunk's scope. A run can only restore caches saved on its own branch or on trunk; the merge queue runs on throwaway gh-readonly-queue branches whose caches die with them and nothing built on trunk, so every merge-queue build started cold and spent 20 to 48 minutes on dependencies (the last run's logs show "Cache not found" on all five platforms, then fresh saves nobody can reach). ci.md explains the scoping.