A second branch needs its own checkout to be worked on in parallel, with its own
slot, subnet, ports and container names so two deploys do not take each other's.
The two skills route to the make targets that create and release one.
The sandbox gains the worktree directory as a writable path, without which the
checkout cannot be created and the skills are unusable. It is the only change
to the permission file: one write path, no deny or ask rule touched.
