- Add -t FLAG to specify custom toplevel (arity 0 predicate)
- Default toplevel is 'repl' if -t is not specified
- Using `-t halt` achieves original goal of guaranteed termination
- Custom toplevels enable flexible exit strategies (e.g., server mode)
- Update help text to document -t flag
Examples:
scryer-prolog -t halt program.pl # Exits after execution
scryer-prolog -t my_repl program.pl # Custom REPL
scryer-prolog program.pl # Default REPL
Co-Authored-By: J.J.'s Robot <noreply@example.com>
