Suggested test: copilot/custom-engine-threat-detection

## Motivation

Link to the gh-aw PR: github/gh-aw#59636 — Fix threat detection config_error for workflows using custom engines

## Proposed test

- **Workflow file**: `test-copilot-custom-engine-threat-detection.md`
- **Trigger**: `workflow_dispatch`
- **Engine**: custom `engine.id` (non-built-in) with `detection-engine: copilot`
- **Safe output**: `create-issue`
- **Variant**: standard

## Minimal test prompt sketch

Define a workflow using a custom engine definition with `detection-engine: copilot`, plus a trivial `create-issue` safe output with a `samples:` block. Assert the compiled `.lock.yml` threat-detection step passes `--engine copilot` (not the custom ID), and that the run completes without `THREAT_DETECTION_STATUS: reason=config_error`.

## New fixtures or secrets needed

Needs a custom engine definition usable in this repo (no such fixture currently exists here) — flagging for triage on whether a minimal custom engine can be added safely.

## Notes

No existing test uses a custom `engine.id` or `detection-engine` key. This is compiler/threat-detection plumbing, verifiable from the compiled lockfile with `--use-samples`, so no live custom-engine credentials should be required.

> Generated by [🔍 Suggest New E2E Tests](https://github.com/githubnext/gh-aw-test/actions/runs/34456199311) · copilot · auto · 54.6 AIC · ⌖ 6.16 AIC · ⊞ 8.4K · [◷](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-custom-engine-threat-detection.md workflow and review how its compiled .lock.yml output represents the threat-detection step. Determine whether a minimal custom engine fixture can be added safely, then verify with --use-samples that the step uses --engine copilot and avoids the config_error status.

Written by the indexing model from the issue text.