Suggested test: copilot/engine-copilot-sdk-mode

## Motivation

Link to the gh-aw PR: github/gh-aw#48978 — "Mark copilot-sdk mode as stable". `engine.copilot-sdk: true` no longer emits an experimental warning at compile time and is now documented as a first-class Copilot variant (SDK mode). No existing test exercises this engine mode.

## Proposed test

- **Workflow file**: `test-copilot-sdk-mode.md`
- **Trigger**: `workflow_dispatch`
- **Engine**: copilot, with `engine.copilot-sdk: true`
- **Safe output**: `create-issue` (simple, easy to assert)
- **Variant**: standard

## Minimal test prompt sketch

Compile and dispatch a minimal workflow with `engine: { id: copilot, copilot-sdk: true }` that asks the agent to create an issue with a fixed title/body via a `samples:` block, confirming the SDK-mode engine path compiles and runs like the standard Copilot CLI mode.

## New fixtures or secrets needed

None — reuses the standard `githubnext/gh-aw-test` fixture and existing Copilot credentials.

## Notes

Distinct from all existing `test-copilot-*` tests, which use the default (non-SDK) Copilot engine. Verify with maintainers whether `copilot-sdk: true` requires any additional runner setup beyond the standard Copilot engine bootstrap in `copilot-setup-steps.yml`.

> Generated by [🔍 Suggest New E2E Tests](https://github.com/githubnext/gh-aw-test/actions/runs/30530172057) · aut00 · 33.4 AIC · ⌖ 6.57 AIC · ⊞ 8.3K · [◷](https://github.com/search?q=repo%3Agithubnext%2Fgh-aw-test+is%3Aissue+%22gh-aw-workflow-call-id%3A+githubnext%2Fgh-aw-test%2Fsuggest-new-e2e-tests%22&type=issues)

Start with the proposed test-copilot-sdk-mode.md workflow and compare the existing test-copilot-* tests for their standard engine setup. Check copilot-setup-steps.yml and the standard githubnext/gh-aw-test fixture before dispatching the workflow. Done means the workflow compiles and runs with engine.copilot-sdk: true, creates the fixed test issue, and confirms the SDK-mode path.

Written by the indexing model from the issue text.