pr-shepherd

Drives an open pull request to landed through an event-driven loop. On each wake it does one batched read, fixes root causes of red required checks, dispositions every review thread, and lands at the level set by the autonomy dial. It pushes and can merge, so it runs only on /lever shepherd, a hand-off from a lever coordinator, a PR labeled for shepherding, or an explicit ask ("monitor CI until green", "push fixes until green", "resolve the review threads", "merge when green", "babysit this PR", "shepherd