Add a citations capability to the model schema

Problem

The schema can represent attachments, reasoning, tool calls, structured output, temperature, and modalities, but it cannot say whether a model returns source citations.

This is a model/API capability, not just a UI label. Consumers need it to decide whether citation options are valid and whether citation-bearing response fields should be expected.

Concrete examples

• Perplexity Sonar always returns sources in top-level citations and search_results: https://docs.perplexity.ai/docs/sonar/prompt-guide
• The Perplexity API response schema exposes citations: https://docs.perplexity.ai/api-reference/sonar-post
• Anthropic's current Models API reports a citations.supported capability per model.
• OpenAI search models return URL citation annotations.

RubyLLM previously maintained local model-ID rules for this fact. During an audit we removed those rules because broad family matching becomes stale and can assign unrelated metadata to new snapshots. There is currently no models.dev field to replace the citation facts.

Suggested shape

A top-level optional boolean would match the existing capability fields:

citations = true

It should be allowed in model metadata and provider overrides because citation support can depend on the serving API, not only the underlying model.

Locate the existing top-level capability fields in the model metadata schema and the provider override definitions. Follow how a boolean capability is validated and represented, then add citations support for both locations and verify that model metadata and provider overrides accept the suggested citations = true shape.