feat(scanner): detect Python package managers

[Will-thom]

Fixes #21.\n\nAdds scanner support for Poetry, PDM, and uv Python package manager signals. The scanner now detects their lockfiles and pyproject tool sections, infers package-manager-specific install/test/lint/format commands, and documents the updated detector coverage.\n\nValidation:\n- npm test\n- node --check src/scanner.mjs\n- node --check test/scanner.test.mjs\n- git diff --check\n- node bin/agent-ready.mjs doctor\n\nNote: npm run check could not be executed through npm in this local Windows environment because npm/Node attempted to access a denied path at C:\Users\itdev\Desktop before running the script. The touched JavaScript files were checked directly with node --check.

[Will-thom]

I checked this against the current main while preparing to resolve the conflict. The Python package-manager detection from this PR appears to have been superseded by the main branch already: main now includes Poetry/PDM/uv lockfile and pyproject detection, package-manager-specific install/test/lint/format commands, docs updates, and renamed fixtures for python-poetry-app, python-pdm-app, and python-uv-app.

Because of that, I did not force-push a rebased branch that would either duplicate the current implementation or turn the PR into an empty diff. Happy to close this if you agree it is now covered by main.