ymm-options

When the user wants to browse available years, makes, models, trims, or variants (dropdowns, "what years was X sold", "which models does Toyota have in 2023"):

1. Make an HTTP GET request to the CarsXE YMM Options API. Include only filters the user provided:

   GET https://api.carsxe.com/v1/ymm-options?key={CARSXE_API_KEY}&source=codex_plugin[&dimension={DIMENSION}][&year={YEAR}][&make={MAKE}][&model={MODEL}][&trim={TRIM}]

dimension is optional: years | makes | models | trims | variants.

make is required for dimension=models. model is required for dimension=trims, and for dimension=variants unless both year and make are set. trim is a substring filter (ignored for years/makes/models).

1. If dimension is omitted, infer the next list: no filters → years; year → makes; make → models; make + model → variants.
2. Present the single returned list. Show message if present. For bulk variants (dimension=variants + year + make, no model), mention modelCount — that is the billed unit count — and warn before making that call.
3. Offer to fetch the next dropdown level or to run the year-make-model skill (/v1/ymm) for full specs of a chosen year/make/model.
4. If the API key is missing, tell the user to set the CARSXE_API_KEY environment variable (see AGENTS.md).