Scratch Obeyas pick their own port

Coding agents stage a throwaway Obeya instance for every check and every demo take. Until now each had to pick a port by hand, and two agents picking the same one at once made the staging fail. The staging script now picks a free port itself and prints it, so agents working in parallel never collide. Staging again from the same stage file reuses its port and stops the previous instance, so repeated demo takes leave nothing running.