CI: refuse to merge while a PR named under a "Depends on" heading is still open

Time: ~2-3 h · Difficulty: Intermediate · Area: .github/workflows/

What happens

A PR body can carry a Depends on section listing other PRs that must land first. GitHub acts on Closes #N and does nothing with Depends on, so today the order is a reviewer's memory. It slipped once already: #3528 (the provider-authoring guide) merged while #3513, which it lists as landed, was still open, so for a while the guide described helpers that were not on main.

What we want

A required check that:

1. reads the PR body, finds a Depends on heading (or a Depends on: line) and every #N under it;
2. resolves each #N through the API;
3. fails while any referenced PR is still open, and passes once all are merged or closed.

Same shape as the existing PR-body checks. It clears itself when the dependencies land, so nobody has to re-trigger anything.

Acceptance

• A PR whose body lists an open PR under Depends on shows a failing check that names it.
• Merging or closing that PR turns the check green on the next run, with no manual step.
• A PR with no Depends on section is unaffected.
• The workflow needs pull-requests: read only.

Proposed by @ilyakanevskiy in #3528.