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 

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[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 on your machine then push the changes, as currently the generated files are out of date.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!