Package app is oc-transcript itself: the flags, the store, the rendering and
the tail. The root main.go is a shim over Main below, and carries the
command's own documentation, where `go doc` looks for it.
Main is the command: it runs the line it was given and sets the exit status.
A command line pflag itself rejects — an unknown flag, a missing value —
exits 2, the way argparse does; a flag whose value this package rejects, and
every other failure, exits 1.