Mark stacked PRs so reviewers can filter the PR list #1945

Part of #1923. The measurement half is in #1924; this issue covers the reviewer-facing half, delivered by
#1925, so each PR is judged against the requirement it actually implements.

Requirement

A reviewer cannot tell a stacked unit apart from a standalone PR in the PR list. The reconcile workflow
already reads PR metadata, so it can identify a unit and apply a stacked label for filtering.

• A stacked unit is a head commit with exactly one parent, and that parent is another open PR's head.
• stacked is a label definition, reconciled separately from the review-state labels, so reconcileLabels
  never removes it; a stale label is removed when the parent is no longer an open PR head.
• A failed open-PR map lookup warns and skips label mutation for that run rather than deciding from an
  empty map, so review-state reconciliation still runs.
• Detection stays in the commit graph for the mutation gate; the label exists only so reviewers can
  filter, and must not be read as a detection signal.

PR: #1925