Function fetchGhPRStack

packages/shared/pr-github.ts:724–799

(
  runtime: PRRuntime,
  ref: GhPRRef,
  metadata: PRMetadata,
)

 * Down: walk from currentPR.headBranch → leaf PRs (descendants)
 */
export async function fetchGhPRStack(
  runtime: PRRuntime,
  ref: GhPRRef,
  metadata: PRMetadata,
): Promise<PRStackTree | null> {
  if (metadata.platform !== "github") return null;
  const defaultBranch = metadata.defaultBranch;
  if (!defaultBranch) return null;

  const currentNode: PRStackNode = {
    branch: metadata.headBranch,
    number: metadata.number,
    title: metadata.title,
    url: metadata.url,
    isCurrent: true,
    isDefaultBranch: false,
  };

  // Walk up: find the PR whose headRefName === baseBranch, repeat
  const ancestors: PRStackNode[] = [];
  let nextHead = metadata.baseBranch;
  const maxDepth = 10;

  for (let i = 0; i < maxDepth; i++) {
    if (nextHead === defaultBranch) break;

    const prs = await queryPRsByRef(runtime, ref, "head", nextHead);
    if (prs.length === 0) {
      ancestors.push({ branch: nextHead, isCurrent: false, isDefaultBranch: false });
      break;
    }

    const pr = prs[0];
    ancestors.push({
      branch: pr.headRefName,
      number: pr.number,
      title: pr.title,
      url: pr.url,
      isCurrent: false,
      isDefaultBranch: false,
      state: (pr.state === 'MERGED' ? 'merged' : pr.state === 'CLOSED' ? 'closed' : 'open') as PRStackNode['state'],
    });
    nextHead = pr.baseRefName;
  }

  // Walk down: find PRs whose baseRefName === current headBranch, repeat
  const descendants: PRStackNode[] = [];
  let nextBase = metadata.headBranch;

  for (let i = 0; i < maxDepth; i++) {
    const prs = await queryPRsByRef(runtime, ref, "base", nextBase);
    if (prs.length === 0) break;

    const pr = prs[0];
    descendants.push({
      branch: pr.headRefName,
      number: pr.number,
      title: pr.title,