Stacked PRs

Topology comes from base branches

PR B is a child of PR A when B.base.ref == A.head.ref. Never infer the stack from titles, labels, or description conventions — those drift, base branches do not.

Halt and report on:

• Cycles — two PRs based on each other's heads.
• Orphans — the base branch no longer exists, usually because a parent merged and its branch was deleted while a grandchild still pointed at it.
• Cross-stack dependencies — PR X logically needs PR Y, but Y is not X's ancestor. This cannot be expressed in base branches; record it as a landing-order constraint instead.

After a parent lands