mirrorball: SSH port forwarding you can type from memory

Field notes

The utility preserves the user’s existing SSH setup, including `~/.ssh/config` aliases, ProxyJump, agent forwarding, and hardware keys, rather than introducing a separate remote agent. Its syntax covers both simple port lists and mappings such as `8080:db.internal:5432`, while background sessions expose `ls`, `logs`, and `stop` commands. A probe distinguishes a service that is actually reachable from one whose local tunnel merely accepted a socket.