feat: add Vercel AI Gateway as a supported model provider

[Sayt-0]

Summary

Adds first-class support for Vercel AI Gateway as an OpenAI-compatible provider via the alias map. Vercel AI Gateway exposes a single unified endpoint (https://ai-gateway.vercel.sh/v1) routing to OpenAI, Anthropic, Google, xAI and more, authenticated with AI_GATEWAY_API_KEY.

Implementation mirrors the recently merged Moonshot provider (17a6e5ec), with one deliberate difference: Vercel is classified as a multi-provider gateway (coalesces consecutive system messages, like openrouter) rather than a first-party API, since it can front open-weight models with strict chat templates (issue #3344).

Definition of Done

Item	Status	Where
Alias entry vercel (openai / base URL / AI_GATEWAY_API_KEY)	done	pkg/model/provider/aliases.go
Unit test: alias resolves and IsCatalogProvider("vercel") is true	done	pkg/model/provider/aliases_test.go (TestCatalogAliases)
Provider appears in CatalogProviders()	done	covered by existing EachAlias-driven TestCatalogProviders
task test and task lint pass	done	see Validation below
Example YAML config	done	examples/vercel.yaml

Additional wiring (matches the Moonshot precedent)

Area	Files
Gateway classification (system-message merge)	pkg/model/provider/openai/client.go, openai/system_message_merge_test.go
End-to-end and opt-in live API test	pkg/model/provider/openai_alias_providers_test.go
Auto model selection	pkg/config/auto.go, pkg/config/auto_test.go
Example parsing (skip models.dev lookup for the router)	pkg/config/examples_test.go
Schema and docs	agent-schema.json, docs/_data/nav.yml, docs/concepts/models/, docs/configuration/models/, docs/providers/overview/, docs/providers/vercel/index.md, examples/README.md

Tests

Coverage is added as rows in the existing table-driven harnesses rather than new bespoke files. The shared alias harness gives both a hermetic end-to-end request (auth header, endpoint, model, SSE reassembly, system-message coalescing) and an opt-in live API check:

AI_GATEWAY_API_KEY=... go test -run TestOpenAIAliasProvider_LiveAPI ./pkg/model/provider/

Validation

Check	Result
go build ./...	pass
targeted tests (pkg/model/provider/..., pkg/config/...)	pass, all vercel subtests green
golangci-lint run	0 issues
custom go run ./lint .	no offenses (1330 files)

Design note

Auto-selection wiring and the default model (vercel/openai/gpt-5, placed moonshot > vercel > amazon-bedrock in the priority order) go beyond the strict DoD, added for parity with the Moonshot provider. If keeping Vercel as an alias-only provider (resolvable but not auto-selected, like requesty) is preferred, the pkg/config/auto.go and auto_test.go changes can be dropped.