Check both branch deltas before reusing a long-lived release PR

A long-lived release PR should be checked in three directions before reuse: the intended source-only release delta, target-only divergence, and a read-only three-way conflict preview. This applies when release branches preserve merge ancestry and an open source→target PR already exists; it does not replace the repository’s prescribed merge strategy or authorize conflict resolution. The source-only delta explains what the release introduces, while target-only commits and the merge preview explain why an otherwise green PR may no longer be mergeable. Avoid opening a duplicate PR when one already exists, and do not treat green checks as evidence that branch ancestry is conflict-free.