dsh-merging-stacked-prs

# Landing an official GitHub PR stack Land dependent PRs through GitHub's native stack object and `gh stack merge`. Do not reproduce stack semantics by merging and retargeting individual PRs with `gh pr merge` and `gh pr edit`. The root [AGENTS.md](../../../AGENTS.md) owns the allowed merge-forward and rebase histories; the [stack review guide](../../../docs/cookbook/responding-to-pr-review-on-a-stack.md) owns review-fix propagation. ## Require native stack support Run `gh stack --version` before changing GitHub state. Hard-stop if the official extension or server-side stack feature is unavailable; do not fall back to manually merging and retargeting PRs one at a time. GitHub stacks require every head branch to live in the same repository, so hard-stop on a cross-fork chain. Use a clean dedicated worktree. Fetch current PR metadata and exact head OIDs rather than trusting branch names or an earlier report: ```sh gh pr view <pr> --json number,author,baseRefName,baseRefOid,headRefName,headRefOid,isCrossRepository,state,isDraft,reviewDecision,mergeStateStatus,statusCheckRollup ``` Query `PullRequest.stack` and `stackEntry.position` for at least one PR in each apparent chain; this official GitHub object, not base-branch inference alone, is the stack-membership authority. Paginate `entries` when `size` exceeds the returned page: ```sh gh api graphql -F owner=<owner> -F name=<repo> -F number=<pr> -f query=' query($owner: String!, $name: String!, $number: Int!) { repository...