Renovate PRs accumulate unmerged when CI runs before the merge-renovate
job was added, or when the automerge label is missing from the repo.
Add _open_renovate_prs() and a new catch-up section (2c) that detects
open renovate/* PRs with passing CI and merges them automatically.
Also created the missing 'automerge' label so Renovate can tag future
minor/patch PRs correctly for the merge-renovate CI job.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
