feat: add dns command

[TacoRocket]

Summary

• add a new operator-first dns command for public and private Azure DNS zone inventory
• wire dns through CLI, help, table/csv output, models, schemas, fixtures, and golden coverage
• document deferred dns follow-on depth in the future roadmap parking lot

Testing

• python3 -m ruff check src tests scripts
• PYTHONPATH=src python3 scripts/generate_schemas.py
• PYTHONPATH=src python3 -m pytest -o cache_dir=/private/tmp/pytest-cache-dns tests/test_collectors.py tests/test_golden_outputs.py tests/test_contract_schemas.py tests/test_cli_smoke.py tests/test_help.py tests/test_terminal_ux.py tests/test_models.py
• PYTHONPATH=src python3 -m pytest -o cache_dir=/private/tmp/pytest-cache-all