Suggested test: copilot/update-pull-request-replace-island

## Motivation

Link to the gh-aw PR: github/gh-aw#54961 — Expose `replace-island` operation for `update-pull-request` safe-output, enabling deterministic marker-delimited section updates to PR bodies instead of full-body regeneration.

## Proposed test

- **Workflow file**: `test-copilot-update-pull-request-replace-island.md`
- **Trigger**: `pull_request` (opened/reopened), following the pattern of `test-copilot-update-pull-request.md`, gated with an `if:` marker check on the PR body so it's only exercised for intentionally-tagged test PRs
- **Engine**: copilot
- **Safe output**: `update-pull-request` with `operation: replace-island`
- **Variant**: standard (uses the main `gh-aw-test` fixture repo, no new fixtures needed)

## Minimal test prompt sketch

Have the agent replace a marker-delimited "island" section of the PR body (e.g. content between `` / `` markers) with new deterministic text, leaving the rest of the body untouched. A `samples:` block can supply the exact island replacement text so the test runs deterministically with `--use-samples`.

## New fixtures or secrets needed

None. Reuses `githubnext/gh-aw-test` and the existing `update-pull-request` PR-opening pattern already used by `test-copilot-update-pull-request.md`.

## Notes

Not covered by any existing test — `test-copilot-update-pull-request.md` only exercises `title`/`body`/`footer` fields, not the new `operation: replace-island` config. Not present in `fails.txt` or `open-suggestions.json`. This is new since `replace-island` was previously only available for `update-issue`, not `update-pull-request` (see github/gh-aw#54906).

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