fix(ticket-005): support coverage.py 7.x contexts, resilient pytest runner and src pythonpath

[tom-sapletta-com]

Summary

• Fix load_coverage_json to support real coverage.py 7.x schema with line-number keys ({line_str: [contexts]}) alongside legacy mock schema ({context: [lines]}), resolving inverted mapping where lines became tests and tests became lines.
• Make run_pytest resilient: detect if pytest-json-report is installed before passing --json-report, and run coverage json --show-contexts with isolated COVERAGE_FILE so coverage.json always contains line contexts without SQLite context table collisions.
• Add pythonpath = ["src"] to [tool.pytest.ini_options] in pyproject.toml so tests run seamlessly without manual PYTHONPATH exports.
• Add __main__.py entrypoint and executable guard in cli.py.
• All 49 unit tests pass, and testless scan, testless duplicates, testless doctor and testless planfiles work end-to-end.