Rotating logs a running process holds: stop it, mv, recreate 664

To give a demo or benchmark a clean log while keeping the old one, renaming the file is not enough while the process runs: it holds the open file descriptor and keeps appending to the renamed file. Stop the processes first, then mv log log.<timestamp>, then create the new empty file yourself (: > f; chmod 664 f) before restarting. If you let the program create the file instead, a hardcoded 0644 locks a teammate's account out of appending on shared hosts, and their process fails at startup. Renaming someone else's file needs write permission on the directory, not ownership of the file. In tmux-run services, a pipe-pane console mirror also removes the need for remain-on-exit: the startup error survives in the file, so the session can close with the process instead of leaving a dead session that blocks the next start.