Add visual regression reference prompt (.github/aw/visual-regression.md) (#18272)

Author: Copilot

.github/aw/create-agentic-workflow.md

@@ -208,7 +208,7 @@ These resources contain workflow patterns, best practices, safe outputs, and per
      - `Package.swift`, `*.podspec` → add `swift`
      - `composer.json` → add `php`
      - `pubspec.yaml` → add `dart`
-   - 💡 If you detect the task requires **browser automation**, suggest the **`playwright`** tool.
+   - 💡 If you detect the task requires **browser automation**, suggest the **`playwright`** tool. For **visual regression testing** (comparing screenshots across PRs), consult `.github/aw/visual-regression.md` for the reference pattern using `playwright` + `cache-memory`.
    - 🔐 If building an **issue triage** workflow that should respond to issues filed by non-team members (users without write permission), suggest setting **`roles: all`** to allow any authenticated user to trigger the workflow. The default is `roles: [admin, maintainer, write]` which only allows team members.
 
    **Scheduling Best Practices:**

.github/aw/visual-regression.md

@@ -0,0 +1,58 @@
+---
+name: visual-regression
+description: Reference prompt for visual regression testing using playwright + cache-memory for baseline screenshot storage across pull requests
+---
+
+# Visual Regression Testing
+
+Use `playwright` for screenshots and `cache-memory` to persist baselines between PR runs.
+
+## Example Workflow
+
+```markdown
+---
+description: Capture screenshots on every PR and compare against cached baselines to detect visual regressions
+on:
+  pull_request:
+    types: [opened, synchronize, reopened]
+permissions:
+  contents: read
+  pull-requests: read
+engine: copilot
+tools:
+  playwright:
+    allowed_domains:
+      - localhost
+      - 127.0.0.1
+  cache-memory:
+    key: visual-regression-baselines-${{ github.event.pull_request.base.ref }}
+    retention-days: 30
+    allowed-extensions: [".png", ".json"]
+  bash:
+    - "mkdir *"
+    - "cp *"
+    - "diff *"
+    - "date *"
+safe-outputs:
+  add-comment:
+    max: 1
+timeout-minutes: 30
+---
+
+Build and serve the app locally, then use Playwright to capture full-page screenshots of key pages into `/tmp/visual-regression/current/`.
+
+Use filesystem-safe timestamps (no colons — colons break artifact uploads):
+`date -u "+%Y-%m-%d-%H-%M-%S"`
+
+If `/tmp/gh-aw/cache-memory/baselines/manifest.json` does not exist, copy screenshots there as new baselines and post: "Baselines initialized — N pages captured."
+
+Otherwise compare each screenshot to its baseline. Post a comment summarizing: pages unchanged / pages with diffs. If nothing changed, use the `noop` safe-output.
+```
+
+## Key Design Decisions
+
+- **`cache-memory` key per base branch** — scopes baselines to `main`, `develop`, etc. independently
+- **`allowed_domains: [localhost, 127.0.0.1]`** — prevents SSRF; app must be served locally
+- **`retention-days: 30`** — keeps baselines beyond the default 7-day cache expiry
+- **Filesystem-safe timestamps** — `YYYY-MM-DD-HH-MM-SS` format; colons are invalid in artifact filenames
+- **Minimal permissions** — all PR writes go through `safe-outputs`, not GitHub tools