feat: Add Stack field to PullRequest for stacked pull requests

[joeljeske]

GitHub's REST API and pull_request webhook payloads now include a stack
object on pull requests that belong to a stack (stacked pull requests). It
reports the branch the whole stack ultimately targets, which can differ from the
PR's own base branch. go-github didn't expose it.

This adds PullRequest.Stack and a PullRequestStack type
(id, number, base, size, position). Because webhook events reuse the
PullRequest struct (PullRequestEvent.PullRequest), the field is available for
both REST responses and incoming pull_request webhooks.

Accessors and stringify tests were regenerated via go generate ./....

Ref: pull request schema in the REST API docs
(https://docs.github.com/en/rest/pulls/pulls), which documents the stack
object.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.54%. Comparing base (34349a8) to head (d2ff87e).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4423   +/-   ##
=======================================
  Coverage   97.54%   97.54%           
=======================================
  Files         194      194           
  Lines       19765    19765           
=======================================
  Hits        19279    19279           
  Misses        268      268           
  Partials      218      218           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[gmlewis]

Thank you, @joeljeske!
LGTM.
Awaiting second LGTM+Approval from any other contributor to this repo before merging.

cc: @stevehipwell - @alexandear - @Not-Dhananjay-Mishra

[joeljeske]

Friendly bump @stevehipwell - @alexandear - @Not-Dhananjay-Mishra
This feature is now public, not in private preview anymore: https://github.blog/changelog/2026-07-30-stacked-pull-requests-are-now-in-public-preview/

[Not-Dhananjay-Mishra]

Few nits, otherwise LGTM.
Apologies for the delay 🫠.

[joeljeske]

Thank you @Not-Dhananjay-Mishra great find - completely agree! I applied your suggestions here.

[Not-Dhananjay-Mishra]

Thanks @joeljeske. One more thing, you'll need to run go generate ./... on your machine then push the changes, as currently the generated files are out of date.

[joeljeske]

Ah thanks @Not-Dhananjay-Mishra. Done!