SDK supports model selection

[kinseydurhamgrace]

We need to support model selection (I think optionally) in the new engine SDK. This PR adds support for this.

https://github.com/github/sweagentd/issues/10067

[Copilot]

Pull request overview

Adds first-class “model selection” support to the Copilot Engine SDK, allowing engines (and the local test CLI) to consume platform-selected model metadata and resolve an effective model to use.

Changes:

• Extend JobDetails with model selection fields (features.model_selection, selected_model, default_model, available_models, etc.).
• Add SDK helpers isModelSelectionEnabled and resolveSelectedModel and export them from the package entrypoint.
• Update the integration guide and engine-cli mock platform/runner to surface model selection fields and environment variables.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/index.ts	Exposes model selection helpers/types from the SDK public API.
src/client.ts	Adds job fields plus helper APIs to detect/resolve selected models.
docs/integration-guide.md	Documents new job fields/env vars and shows SDK usage.
cli/internal/server/server.go	Mock platform server optionally returns model selection fields/feature flag.
cli/internal/runner/runner.go	Injects model selection-related env vars into the engine process.
cli/cmd/engine-cli/main.go	Adds CLI flags to control model selection fields and passes them into mock server/runner.

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[josebalius]

This looks good to me in general, but question, how is an engine supposed to describe the types of models it cares about or that it is compatible with?

[kinseydurhamgrace]

@josebalius That is a good question. We still need the notion of "filtering" here so I will think on how we can do that. Thank you!