test: add test-copilot-model-alias-small

> [!CAUTION]
> Protected files were modified in this change.
> This pull request is in `request_review` mode and requires explicit human scrutiny before merge.
>
> Protected files: `.github/workflows/test-copilot-model-alias-small.md`

## Motivation

Adds a regression gate for [github/gh-aw#44288](https://github.com/github/gh-aw/pull/44288), which fixed the Copilot CLI rejecting the `small` model meta-alias as unsupported. The `small` alias resolves through `mini` to `gpt-5-mini` / `haiku` at runtime, but the CLI was previously failing before that resolution could occur.

## Changes

- **`.github/workflows/test-copilot-model-alias-small.md`** — new `workflow_dispatch` test using `engine.model: small`. The agent creates a single issue. A `samples:` block makes it run deterministically with `--use-samples`.

## Notes

No lock file is included — lock files are generated by `gh aw compile` and committed separately per AGENTS.md convention.

Once `small` resolution is confirmed stable in the smoke suite, this test provides ongoing E2E coverage in the nightly matrix.

> [!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/29009122314) · 97.4 AIC · ⌖ 8.19 AIC · ⊞ 5.6K · [◷](https://github.com/search?q=repo%3Agithubnext%2Fgh-aw-test+%22gh-aw-workflow-id%3A+suggest-new-e2e-tests%22&type=pullrequests)

---

> [!NOTE]
> This was originally intended as a pull request, but the git push operation failed.
>
> **Original error:** pushSignedCommits: refusing unsigned push for branch 'test/copilot-model-alias-small-eef532390953f8f8': Signed-commit payload violates file-protection policy (request_review): .github/workflows/test-copilot-model-alias-small.md
>
> **Workflow Run:** [View run details and download bundle artifact](https://github.com/githubnext/gh-aw-test/actions/runs/29009122314)
>
> The bundle file is available in the `agent` artifact in the workflow run linked above.

To create a pull request with the changes:

```sh
# Download the artifact from the workflow run
gh run download 29009122314 -n agent -D /tmp/agent-29009122314

# Fetch the bundle into a temporary ref, then update the local branch
git fetch /tmp/agent-29009122314/aw-test-copilot-model-alias-small.bundle refs/heads/test/copilot-model-alias-small:refs/bundles/create-pr-test-copilot-model-alias-small-eef532390953f8f8-6e3f4919
git update-ref refs/heads/test/copilot-model-alias-small-eef532390953f8f8 refs/bundles/create-pr-test-copilot-model-alias-small-eef532390953f8f8-6e3f4919
git checkout test/copilot-model-alias-small-eef532390953f8f8
# Ensure the working tree matches the updated branch
git reset --hard
# Remove the temporary bundle ref
git update-ref -d refs/bundles/create-pr-test-copilot-model-alias-small-eef532390953f8f8-6e3f4919

# Push the branch to origin
git push origin test/copilot-model-alias-small-eef532390953f8f8

# Create the pull request
gh pr create --title 'test: add test-copilot-model-alias-small' --base main --head test/copilot-model-alias-small-eef532390953f8f8 --repo githubnext/gh-aw-test
```

Start with .github/workflows/test-copilot-model-alias-small.md and review its workflow_dispatch configuration, engine.model: small, and samples block. Use the AGENTS.md convention and gh aw compile guidance, then verify the workflow provides deterministic --use-samples coverage for the small alias and creates a single issue.

Written by the indexing model from the issue text.