On some Python versions, argparse fails to route subcommand tokens when
the parent parser has nargs='?' optional arguments (--continue). The
symptom: 'hermes model' produces 'unrecognized arguments: model' even
though 'model' is a registered subcommand.
Fix: when argv contains a token matching a known subcommand, set
subparsers.required=True to force deterministic routing. If that fails
(e.g. 'hermes -c model' where 'model' is consumed as the session name
for --continue), fall back to the default optional-subparsers behaviour.
Adds 13 tests covering all key argument combinations.
Reported via user screenshot showing the exact error on an installed
version with the model subcommand listed in usage but rejected at parse
time.
