Release Cleanup

Confirm a release branch's work has reached the trunk (default branch), then prune the feature branches and git worktrees that are no longer needed. Verification is a hard gate: never prune until each branch's work is proven to have reached the trunk and no in-flight work is stranded.

This skill is standalone and manually triggerable. It does not promote code (use release-pr-gates for that) and does not deploy (use deploy). It runs after a promotion has landed and tidies up.

The Merge Oracle (read this first)

Commit ancestry is NOT a reliable merge signal. GitHub's default merge mode is squash, which collapses a branch into a single new commit on the base. After a squash merge the branch tip is not an ancestor of the base, so git branch --merged / --no-merged and A..B ranges all report a fully-merged branch as unmerged. Rebase-merges have the same property.