landing-plan

Turns a set of open PRs into a merge plan. It maps their dependencies, puts the PRs into merge-queue batches with strict order only where they overlap, and writes a hand-off with evidence for each PR. At L2 it gives the plan to the human who will merge; at L4 it enqueues the PRs itself. It can enqueue, so it runs only on /lever land, a hand-off from a lever coordinator, or an explicit ask ("what order should these merge in", "land the stack", "why is landing taking all day").