Add python module entrypoint

[hannibal-lee]

What

Adds a python -m whichllm module entrypoint and documents it as a fallback command for environments where the generated uvx launcher fails before whichllm starts.

Why

On older macOS versions, uvx whichllm@latest can fail before invoking whichllm because the generated launcher depends on realpath, which may not be available:

realpath: command not found
/Users/.../python: No such file or directory

This PR does not change the existing whichllm console script. It adds a portable fallback:

uvx --from whichllm python -m whichllm

## Testing

- [ ] Tests pass (pytest)
- [x] New tests added
- [ ] Tested on real hardware (if hardware-related)

Verified locally with:

PYTHONPATH=/Users/hannibal/whichllm/src /Users/hannibal/.cache/uv/archive-v0/b_ZZfBOyPpAdyWVq/bin/python -m whichllm --version

Output:

0.5.10

Full uv run pytest was blocked locally by network/TLS download failures while installing dependencies.

## Notes

This is a workaround for old launcher environments, not a replacement for fixing launcher generation in uv itself.