[ci] Avoid running full CI mid-stack for GH-native stacks, same as we do for Graphite (#95218)
Running CI only on the top and bottom of the stack avoids expensive
`O(n^2)` behavior when merging one PR at a time and recursively rebasing
all the others starting at the bottom of the stack.
GitHub added this to the webhook/GH actions object for us to be able to
do this: https://vercel.slack.com/archives/C01SKER9S1G/p1777926655162839
Prior to this PR being part of a stack, fails open (runs full CI):
<img width="1938" height="993" alt="Screenshot 2026-06-26 at 3 03 37 PM"
src="https://github.com/user-attachments/assets/54d6055a-2860-45b3-a03a-a9356b3db2a4"
/>
Example mid-stack run (skips the full e2e jobs):
<img width="1917" height="664" alt="Screenshot 2026-06-26 at 3 24 33 PM"
src="https://github.com/user-attachments/assets/9ee466da-f3ea-4298-b5b8-2527d197c121"
/>
https://github.com/vercel/next.js/actions/runs/28268549361/job/83760907003?pr=95219