Add Vercel AI Gateway as a supported model provider

[aheritier]

Summary

Add first-class support for Vercel AI Gateway as an OpenAI-compatible model provider via the alias map.

Why

Vercel AI Gateway provides a single unified endpoint to access models from OpenAI, Anthropic, Google, xAI and more at list price with no markup, plus provider routing/failover. It is popular with web developers and reduces the need to manage many separate keys.

Resources

• OpenAI-compatible base URL: https://ai-gateway.vercel.sh/v1
• Token env var: AI_GATEWAY_API_KEY
• Docs: https://vercel.com/docs/ai-gateway
• Implement in: pkg/model/provider/aliases.go
• Test: pkg/model/provider/aliases_test.go

Definition of Done

• Alias entry added: "vercel": {APIType: "openai", BaseURL: "https://ai-gateway.vercel.sh/v1", TokenEnvVar: "AI_GATEWAY_API_KEY"}
• Unit test added; asserts alias resolves and IsCatalogProvider("vercel") is true
• Provider appears in model catalog via CatalogProviders()
• task test and task lint pass
• Example YAML config using a Vercel AI Gateway model added