Support Github Native Stack PRs

[gabemeola]

Now in public preview

https://docs.github.com/en/pull-requests/how-tos/stacked-pull-requests?utm_source=changelog-stacked-pr-docs&utm_medium=changelog&utm_campaign=stacked-prs-gtm-public-preview-2026

[raydatray]

i vibecoded an implementation here that seems to work pretty well - feel free to try it out !

• [7/7][sl][github][gh stacks] update some documentation #1398
• [6/n][sl][github][gh stacks] wire up gh stacks to sl pull #1397
• [5/n][sl][github][gh stacks] wire up gh stacks to sl pr submit #1396
• [4/n][sl][github][gh stacks] update submit body for stacks #1395
• [3/n][sl][github][gh stacks] add stacked workflow for submit #1394
• [2/n][sl][github][gh stacks] add support for stack endpoint #1393
• [1/n][sl][github][gh stacks] support headers and list params #1392

[martpie]

Very excited to be be able to use Sapling/ISL with GitHub stacks, that should hopefully solve many limitations with the current merge system between pr and ghstack