[P0] Fix packaging & lazy imports so `code2test` runs

[pro-utkarshM]

Problem

code2test --help fails: ModuleNotFoundError: No module named 'code2test' / click. Editable install links the package but dependencies are not installed, and code2test/__init__.py:14 eager-imports code2test.cli.main at top level — any missing dep breaks import code2test entirely.

Tasks

• Make code2test/__init__.py lazy: do not import cli.main at module top. Expose CLI via entry point only / lazy attribute.
• Verify clean install path: pip install -e ".[dev]" in a fresh venv pulls all deps (click, rich, litellm, etc.).
• Pin/verify requires-python against tested interpreters (repo venv is 3.14; pyproject says >=3.12 — confirm support or constrain).
• Smoke test: fresh venv → install → code2test --help exits 0.

Acceptance

code2test --help and code2test test --help run with zero import errors in a fresh venv.