Add pythonclaw --version flag #5

The CLI has no --version / -V flag. It should print __version__ from pythonclaw/__init__.py and exit.

Where: pythonclaw/main.py — _build_parser() adds subcommands; add a top-level --version argument using argparse's action="version".

Acceptance: pythonclaw --version prints e.g. pythonclaw 0.7.1.

Good first issue — small, self-contained, no external dependencies.