Add GPT-6 Luna Decisions provider facts

[softmarshmallow]

Add openai/gpt-6-luna-decisions as a distinct decision offering backed by GPT-6 Luna, with native OpenAI as reference and independently sourced Vercel/OpenRouter bindings. The catalogue can now distinguish decision contracts and rates from chat generation without changing existing cards.

Depends on #16; merge that schema PR first, then retarget this PR to main. Closes #15.

Sources and qualifications:

• OpenAI changelog: public beta endpoint release on 2026-10-06, separate from the underlying model release.
• Native guide, exact API reference, and Standard pricing: predicate/choice/score, input-only rates, native typed choices, refusals and long-context input bands. The guide/reference conflict over hosted image URLs is explicitly qualified; no complete native image transport set or undocumented native score limit is asserted.
• Vercel contract, model page, and public endpoint feed: separate input rate/context evidence and exact compatible contract, including its inline images, 1–255 choices and 2–10 score levels. Generic chat discovery conflicts are documented; unrecorded decision price bands are not copied from OpenAI.
• OpenRouter model, alpha API, and multimodal guide: distinct state, named question objects and noul protocol, with sourced input rates and image envelope. Endpoint availability does not imply native OpenAI SDK compatibility.

No inherited chat rates, output limits, tools, reasoning parameters or privacy eligibility promises. No planned Grida provider binding is published.

Validation: regenerated catalogue, checker, all 290 pytest tests, pinned formatter and full public-boundary/fact diff review pass. All 65 existing cards are unchanged. No paid inference calls.