Feature: AIgateway provider integration (declarative provider)

**What problem would this solve?**

Goose's declarative provider set (`crates/goose-providers/src/declarative/definitions/`) includes aggregators like OpenRouter, Novita, Together, Venice and Vercel AI Gateway, but not [AIgateway](https://aigateway.sh). Users who hold an AIgateway key (one `sk-aig-` key for 1,000+ models from 85+ labs — Claude, GPT-5.4, Gemini, Kimi, GLM, DeepSeek — at pass-through provider pricing) currently have to configure it under the generic custom OpenAI-compatible provider, losing Goose's curated model metadata, pricing, and `/v1/models` discovery.

**Who it affects:** anyone using both Goose and AIgateway; also anyone who follows our catalog/docs into Goose.

**What would a good outcome look like?**

A first-class `aigateway` declarative provider: engine `openai`, base URL `https://api.aigateway.sh/v1/chat/completions`, `AIGATEWAY_API_KEY` env, curated model list with context limits + per-token costs, and `catalog_provider_id: aigateway` so the models.dev registry metadata (our provider entry merged there — anomalyco/models.dev#6361) enriches it.

**Possible approaches**

I had opened PR #11885 before this process existed; it was closed with a request to start here. That PR shows the concrete shape:

- `declarative/definitions/aigateway.json` — 13 curated models with per-token pricing from our live catalog (`GET /v1/models` returns 1,065 models; the JSON carries the top ones)
- `expose_declarative_providers!` macro registration
- `test_aigateway_provider_registry_wiring` test in `crates/goose/src/providers/init.rs` (Gondola pattern: name, default model `zai-org/glm-5.3-flash`, secret key) — passing locally
- AIgateway validation section in `goose-self-test.yaml`
- All CI green on the PR, `cargo test -p goose-providers --lib` 217/217

Trade-offs: none unusual — it follows the exact Novita/Vercel-AI-Gateway pattern. The only open question from the code-review bot was whether `max_tokens` fields in the JSON are useful given `ModelInfo` currently ignores them (existing definitions like Novita's carry the same field — happy to follow whatever the maintainers prefer there).

**Ownership & maintenance:** I'm the founder of AIgateway; we'll maintain the provider definition as our catalog evolves (pricing snapshots come from the same pipeline that publishes aigateway.sh/models).

**Additional context**

- PR with the implementation ready to reopen: aaif-goose/goose#11885 (branch still up)
- Our models.dev registry entry (feeds `catalog_provider_id`): anomalyco/models.dev#6361
- API docs: https://aigateway.sh/docs · catalog: https://aigateway.sh/models

- [x] I have verified this does not duplicate an existing feature request

Le travail se situe dans les définitions déclaratives des providers. Commencez par examiner le modèle existant dans crates/goose-providers/src/declarative/definitions/ pour des providers comme Novita ou Vercel AI Gateway. La tâche principale consiste à créer un nouveau fichier aigateway.json avec les modèles et les tarifs spécifiés, puis à l’enregistrer via la macro expose_declarative_providers!. Exécutez la suite de tests existante, en particulier le test dans crates/goose/src/providers/init.rs, afin de vérifier l’intégration. Le travail est considéré comme terminé lorsque CI passe et que le provider apparaît dans le catalogue.

Rédigé par le modèle d'indexation à partir du texte de l'issue.