Use cached launch path for explicit workspace

[lilly-luo]

Summary

• allow an explicit workspace to use cached launch state when the normalized workspace matches
• retain protection against launching with mismatched workspace state
• add focused coverage proving the cached Codex path skips preflight

Testing

• env -u ENABLE_SMART_ROUTING_V2 .venv/bin/pytest tests/test_cli.py -q
• uv run ruff check src/ucode/cli.py tests/test_cli.py