Dato Agent: add Vercel AI Gateway as a provider #168

Problem

Dato Agent settings offer OpenAI and Anthropic. A project that already pays for Vercel cannot point the agent at that account. Editors then need a separate provider key even when the team already has AI Gateway credits or bring-your-own keys.

Proposal

Add an optional Vercel AI Gateway provider next to the existing ones.

• The administrator creates a key under Vercel → AI Gateway → API keys and selects a model.
• The model list starts with a short recommended set for a CMS content agent (Claude Sonnet 5.5 by default, plus Opus, Haiku, GPT, Gemini, and Grok). A toggle reveals the rest of the tool-capable catalog.
• Requests use the existing client-side MCP runtime through the gateway's Anthropic Messages API, so non-OpenAI models can keep the same approval flow. The OpenAI hosted MCP tool cannot do that for models behind the gateway.
• Usage is billed to the Vercel team's gateway credits, or to provider keys configured on the gateway.

Models that are not Claude are translated through that Messages API. Zero data retention still depends on the selected model's zdr flag.