fix(ci): add explicit pnpm@9 preparation to workaround corepack bug in Node 20.16.0

The baseline allowlist workflow was failing with:
  Error: Cannot find matching keyid

Root cause: Node 20.16.0's bundled corepack has a signature verification bug
when running `corepack enable` that tries to auto-detect and install pnpm.

Fix: Add `corepack prepare pnpm@9 --activate` before pnpm install in both jobs
(gen-matrix and run-allowlisted) to explicitly install pnpm 9 without relying
on corepack's buggy auto-detection.

This is a known issue: 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

@@ -27,6 +27,7 @@ jobs:
       - name: Install deps
         run: |
           corepack enable
+          corepack prepare pnpm@9 --activate
           pnpm i --frozen-lockfile
 
       - name: Generate Playwright selection from CSV
@@ -52,6 +53,7 @@ jobs:
       - name: Install browsers (Playwright)
         run: |
           corepack enable
+          corepack prepare pnpm@9 --activate
           pnpm i --frozen-lockfile
           npx playwright install --with-deps