stack object is webhook-only — expose pull request stack membership on REST & GraphQL

[joeljeske]

Summary

When a pull request is part of a stack, the stack object (e.g. stack.base.ref)
is delivered on webhook payloads but is not retrievable from any read API:

• REST (GET /repos/{owner}/{repo}/pulls/{number}) returns stack: null —
  even for a PR that is a confirmed member of a stack.
• GraphQL has no stack field on the PullRequest type at all (querying it
  errors; introspection lists no such field).
• The only programmatic source of stack data is the undocumented
  cli_internal/pulls/stacks endpoint, and it returns membership only
  ({id, pull_requests}) — not the stack base — so determining "what does this
  stack target" requires a second lookup.

The data clearly exists server-side (it's on the webhook), it just isn't exposed
on demand. Please expose it as a supported, documented field on both APIs.

Repro (gh CLI)

Given a PR (#31 below) that is a confirmed member of a stack:

# Confirm it IS in a stack (internal endpoint — membership only, no base):
gh api repos/OWNER/REPO/cli_internal/pulls/stacks
# => [{"id":22,"pull_requests":[29,30,31]}]

# REST: stack is null even though #31 is a member
gh api repos/OWNER/REPO/pulls/31 --jq '{number, base: .base.ref, stack}'
# => {"number":31,"base":"m-B","stack":null}

# GraphQL: the field does not exist
gh api graphql -f query='{ repository(owner:"OWNER", name:"REPO") {
  pullRequest(number:31) { number stack { base { ref } } } } }'
# => Field 'stack' doesn't exist on type 'PullRequest'

# GraphQL introspection: no stack field on PullRequest
gh api graphql -f query='{ __type(name:"PullRequest") { fields { name } } }' \
  --jq '.data.__type.fields[].name' | grep -i stack
# => (no output)

For contrast, the same PR's webhook payload does include it:

  "pull_request": {
    "base":  { "ref": "m-B" },
    "stack": { "base": { "ref": "master", "sha": "…" } }
  }

Request

Expose the stack object on the read APIs, mirroring the webhook shape:

Expose the stack object on the read APIs, mirroring the webhook shape:

• REST — populate stack on the pull request object (stack.base.{ref,sha}),
  and ideally the member PR numbers.
• GraphQL — add PullRequest.stack { base { ref sha } pullRequests { … } }.

This lets any consumer determine stack membership and target on demand, instead
of depending solely on (push-only, sometimes-absent) webhook delivery.