Squash merges break patch-id branch cleanup, so ask the forge

Merged-branch cleanup based on git cherry or patch-id silently fails in a repo that squash-merges, because a squash rewrites every patch-id. Landed branches then look unmerged forever and are never cleaned up.

Why: release-please plus squash PR titles means the PR title is the release commit, so squash is the norm rather than the exception. In duyet/herdr-desk this left three dead branches looking live, and made git log main..branch report six already-landed commits as pending.

How to apply: treat the forge as the authority, not patch-id. A branch is spent only when it has at least one merged PR and no open PR. The "at least one" clause is the load-bearing part: an empty PR list means unknown, not all merged, and treating it as merged deletes in-flight work that was pushed but not yet proposed — which in a shared working tree can be another agent's branch. Keep git cherry only as a backstop for forge lag.

Skip the base branch, forge-managed release branches, and any branch pattern created at runtime. Pair it with a scheduled workflow, dry-run by default.

A diff against the base is still the tie-breaker when a squash makes hashes diverge: if moving from main to the branch is mostly deletions, the branch is behind, not ahead.