concore watch crashes on a negative --interval #582

Noticed this while poking at `concore watch`.

`--interval` is passed straight through to `time.sleep(interval)` in the refresh loop, no validation anywhere along the way. Pass a negative number and it crashes with an unhandled `ValueError: sleep length must be non-negative` instead of a normal error message. `--interval 0` isn't rejected either, just turns the watch loop into a tight busy-loop.

Every other command in `cli.py` validates bad input and prints `[red]Error:[/red] ...` before exiting, this one just doesn't check.

Should be a quick fix, reject `interval <= 0` before calling `watch_study()`. Good first issue for someone new to the repo.