Add terminal warning, argparse CLI, and Polars scalar examples

- Add terminal detection warning in Worker.run() when stdin is a tty
- Update example worker to use create_argument_parser() for CLI
- Create vgi/examples/scalar_polars.py with Polars-based scalar functions:
  - PolarsUpperCaseFunction: String uppercase via Polars
  - PolarsStringLengthFunction: String length computation
  - PolarsNormalizeFunction: Z-score normalization

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: rustyconover