fix(ci): use pnpm/action-setup instead of corepack to bypass Node 20.16.0 bug

The workflow was failing with corepack signature verification errors:
  Internal Error: Cannot find matching keyid

Root cause: Node 20.16.0's bundled corepack has a known bug with signature
verification that affects both `corepack enable` and `corepack prepare`.

Fix: Replace corepack usage with pnpm/action-setup@v4 action which installs
pnpm 9 directly, bypassing corepack entirely. This is the recommended
approach for CI environments.

Related: nodejs/corepack#358

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: mirqtio

.github/workflows/playwright-baseline-allowlist.yml

@@ -24,11 +24,12 @@ jobs:
         with:
           node-version: '20.16.0' # Match package.json engines.node
 
+      - uses: pnpm/action-setup@v4
+        with:
+          version: 9
+
       - name: Install deps
-        run: |
-          corepack enable
-          corepack prepare pnpm@9 --activate
-          pnpm i --frozen-lockfile
+        run: pnpm i --frozen-lockfile
 
       - name: Generate Playwright selection from CSV
         id: select
@@ -50,10 +51,12 @@ jobs:
         with:
           node-version: '20.16.0' # Match package.json engines.node
 
+      - uses: pnpm/action-setup@v4
+        with:
+          version: 9
+
       - name: Install browsers (Playwright)
         run: |
-          corepack enable
-          corepack prepare pnpm@9 --activate
           pnpm i --frozen-lockfile
           npx playwright install --with-deps