Suggested test: copilot/pull-request-target-ready-for-review

## Motivation

github/gh-aw#49865 — `pull_request_target.types` previously rejected the `ready_for_review` activity type, even though it's valid for `pull_request_target` events (e.g. converting a draft PR to ready). Schema now allows `[opened, ready_for_review, ...]`.

## Proposed test

- **Workflow file**: `test-copilot-pull-request-target-ready-for-review.md`
- **Trigger**: `pull_request_target` with `types: [opened, ready_for_review]`
- **Engine**: copilot
- **Safe output**: `add-comment`
- **Variant**: standard

## Minimal test prompt sketch

On `pull_request_target` events of type `opened` or `ready_for_review`, have the agent add a short comment on the PR noting which activity type triggered the run. Confirms the compiler accepts `ready_for_review` in the trigger schema and the generated workflow correctly reacts to that event.

## New fixtures or secrets needed

None.

## Notes

Not exercised by any existing `test-*` workflow (none currently use `pull_request_target` with `ready_for_review`). This is a schema-validation regression test, so `--workflow-dispatch-only` CI runs will still validate compilation even though the `ready_for_review` trigger itself needs a real draft→ready transition to fire locally.

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