This adds a first_available model selector to the config schema. It enables automatic fallback across multiple model candidates by resolving the first reachable one with configured credentials, including support for local dmr and ollama fallbacks.

The implementation automatically tries each candidate in order until one succeeds based on available credentials. If no candidates can be resolved, it reports grouped missing-credential errors indicating that at least one candidate group is needed. During alias resolution, unresolved selectors are skipped, allowing other aliases to proceed.

The change includes an example configuration, comprehensive tests, and all necessary schema updates.

Validation

go test ./pkg/config/... ./pkg/teamloader/... ./pkg/runtime/...
task build
task lint