Suggested test: copilot/close-issue-duplicate-of

## Motivation

Link to the gh-aw PR: github/gh-aw#43152 — feat(close-issue): add `duplicate_of` for native GitHub duplicate marking

The `close-issue` safe output gained a new optional `duplicate_of` field that calls the GitHub `markAsDuplicate` GraphQL mutation to create the native timeline relationship between two issues. There is no E2E test covering this new field.

## Proposed test

- **Workflow file**: `test-copilot-close-issue-duplicate-of.md`
- **Trigger**: `workflow_dispatch`
- **Engine**: copilot
- **Safe output**: `close-issue` (with `duplicate_of` field)
- **Variant**: standard

## Minimal test prompt sketch

The workflow is triggered via `workflow_dispatch`. The agent is given a fixed issue number (a permanent fixture issue in `githubnext/gh-aw-test`, e.g. issue #1) and instructed to close it as a duplicate of another fixed issue (e.g. issue #2), using `state_reason: duplicate` and `duplicate_of: 2`. The test asserts the issue was closed with a `duplicate` state reason.

## New fixtures or secrets needed

None. The test can reuse the existing permanently-closed fixture issues in `githubnext/gh-aw-test`. A `samples:` block can be added to avoid real engine calls in deterministic mode.

## Notes

- Existing `test-copilot-close-issue.md` only exercises plain close (no `duplicate_of`). This test covers the new field introduced in github/gh-aw#43152.
- The `markAsDuplicate` mutation failure path is non-fatal per the PR — the test only needs to verify happy-path emission of `duplicate_of` in the safe output.
- Should eventually be added for claude and codex variants as well, but copilot is sufficient for initial coverage.

> [!WARNING]
>
> Firewall blocked 1 domain
>
> The following domain was blocked by the firewall during workflow execution:
>
> - `awmgmcpg`
>> To allow these domains, add them to the `network.allowed` list in your workflow frontmatter:
>
> ```yaml
> network:
> allowed:
> - defaults
> - "awmgmcpg"
> ```
>
> See [Network Configuration](https://github.github.com/gh-aw/reference/network/) for more information.
>
>

> Generated by [🔍 Suggest New E2E Tests](https://github.com/githubnext/gh-aw-test/actions/runs/28701594992) · 66.9 AIC · ⌖ 6.82 AIC · ⊞ 5.6K · [◷](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 by reading test-copilot-close-issue.md and use it as the model for test-copilot-close-issue-duplicate-of.md. Configure the workflow_dispatch trigger, copilot engine, close-issue safe output, and duplicate_of scenario with the existing githubnext/gh-aw-test fixture issues. Done means the test verifies the issue is closed with a duplicate state reason and can use a samples block for deterministic runs.

Written by the indexing model from the issue text.