[2/n][sl][github][gh stacks] add support for stack endpoint

[raydatray]

ctx

basic plumbing for github native stacks REST API

changes made

• gh_submit: StackDetails dataclass + get_stack_for_pull_request, create_stack, add_prs_to_stack, unstack, all pinned to the 2026-03-10 preview API version
• update_pull_request now takes base: Optional[str]: github rejects updatePullRequest mutations that include baseRefName for PRs in a native stack, so base=None uses a new mutation variant (GRAPHQL_UPDATE_PULL_REQUEST_NO_BASE) that only touches title/body

test plan

CI, and dogfooded on my own computer via a patch raydatray/rusty-mcrouter#198

---

Stack created with Sapling. Best reviewed with ReviewStack.

• [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

[meta-codesync]

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