Never condense into an inherited trailer

An inherited trailer whose checkpoint is not in this clone's store (a teammate's squashed branch, a dangling trailer) looked like a fresh one to post-commit, so declining the link at commit -m still wrote the session's work under the teammate's checkpoint ID. prepare-commit-msg now records the trailers it inherited in the per-worktree git dir, tied to the commit's parent; post-commit drops them before choosing where to condense and consumes the marker.