[Bug] Fix setuptools packaging: include all lcb_runner subpackages - #156

A non-editable pip install of this repo yields a broken lcb_runner package: only
the top-level module is shipped because pyproject.toml pins
packages = ["lcb_runner"]. All subpackages (benchmarks, evaluation, prompts,
runner, utils) are missing, so the harness is unusable for anyone installing
non-editable (pip install . / wheel / sdist / container images). The README's
editable -e . flow masks the issue.

Changes

• pyproject.toml: replace the explicit package list with
  [tool.setuptools.packages.find] include = ["lcb_runner*"] so all subpackages are
  discovered and shipped. No runtime code changes.

Test Plan

• Non-editable install: pip install . (or uv pip install .).
• python -c "import lcb_runner.evaluation.testing_util" succeeds.
• python -m lcb_runner.runner.main --help is reachable.
• Judge smoke: lcb_runner.evaluation.compute_code_generation_metrics.check_correctness
  on a call-based and a stdio sample (correct solution passes, wrong solution fails).

Test Result

• After the fix, site-packages/lcb_runner/ contains benchmarks, evaluation,
  prompts, runner, utils.
• Imports and judge path verified on Python 3.12.