pkgproxyctl: show positional arguments in help, use one usage layout everywhere

pkgproxyctl cache invalidate --help and offline mode --help list only -socket-path and never mention URL or MODE; offline -h says COMMAND though the command is optional; commands that take no arguments ignore stray ones silently. README and DEPLOYMENT.md also present cache invalidate as general, but it only handles apt URLs.

Goal: every command renders its usage through go-cmdr's flagcmd FlagUsageContext in one layout (Usage: X [OPTIONS] ARGS, a short description, Available options:), positional arguments are named and described, wrong argument counts print the usage and a "cannot ..." error, and commands with no arguments reject stray ones. cache invalidate is documented as apt-only, with what to do for the other ecosystems instead.