Suggested test: copilot/cache-memory-concurrent-merge

## Motivation

Link to the gh-aw PR: github/gh-aw#60663 — concurrent repo-memory pushes previously resolved conflicts by always keeping the local version, silently discarding `.jsonl` rows added by a concurrent run. The fix configures Git's union merge driver (`*.jsonl merge=union`) for cache-memory files so concurrent appends are preserved.

## Proposed test

- **Workflow file**: `test-copilot-cache-memory-concurrent-merge.md`
- **Trigger**: `workflow_dispatch`
- **Engine**: copilot
- **Safe output**: `create-issue` (reports the observed merge outcome)
- **Variant**: standard

## Minimal test prompt sketch

Dispatch the workflow twice in close succession with distinct inputs. Each run appends a uniquely-tagged row to a shared `cache-memory` JSONL file, then a follow-up run reads the file and creates an issue reporting whether both tagged rows are present — verifying the union-merge policy preserved both concurrent appends instead of one clobbering the other.

## New fixtures or secrets needed

None — builds on the existing `cache-memory: true` tooling already used by `test-copilot-restore-memory-custom-job.md`, which only covers restore-step injection, not concurrent push/merge behavior.

## Notes

Open question: reliably forcing a genuine write race within the serial E2E harness may need two near-simultaneous dispatches or a two-phase design. Filing as an issue pending a decision on how to force the race deterministically in CI.

> Generated by [🔍 Suggest New E2E Tests](https://github.com/githubnext/gh-aw-test/actions/runs/34824047997) · copilot · auto · 99.5 AIC · ⌖ 4.84 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 test-copilot-cache-memory-concurrent-merge.md and compare it with test-copilot-restore-memory-custom-job.md, especially the existing cache-memory setup. Determine how the serial E2E harness can perform two near-simultaneous workflow_dispatch runs, then verify that the follow-up run reports both uniquely tagged JSONL rows. Done means the test reliably exercises the concurrent merge and confirms neither append is lost.

Written by the indexing model from the issue text.