Move cmd-mox behind a CommandRunner port (#62)

[lodyai]

Summary

This branch moves the cmd-mox IPC integration for issue #62 out of production modules and behind a CommandRunner port selected at the CLI boundary.

Closes #62.

Production code now depends on the runtime command-runner protocol and default subprocess adapter, while the cmd-mox adapter lives under lading.testing and is loaded only when LADING_USE_CMD_MOX_STUB is enabled.

Review walkthrough

• Start with lading/runtime/runner.py and lading/runtime/subprocess_runner.py for the production port and subprocess adapter.
• Then review lading/testing/cmd_mox_runner.py for the test-only cmd-mox IPC and passthrough adapter.
• Review lading/cli.py and lading/workspace/metadata.py for runner selection and cargo metadata routing.
• Finish with tests/unit/test_cmd_mox_integration.py, tests/unit/test_cargo_metadata_loading.py, and tests/bdd/steps/test_publish_infrastructure.py for the updated test coverage.

Validation

• make check-fmt: passed.
• make lint: passed with Ruff clean and Pylint at 10.00/10.
• make typecheck: passed with ty check clean.
• make test: passed, 465 tests and 14 snapshots.
• grep -R "cmd_mox" -n lading: passed with hits only under lading/testing/cmd_mox_runner.py.
• coderabbit review --agent: passed with zero findings after follow-up fixes.