ci(a11y): bump Pa11y workflow Node 20 -> 22 (cspell@10 engine requirement)

The a11y (Pa11y live) PR check failed at "Install root JS deps": root `npm ci`
hits EBADENGINE because @cspell/cspell-*@10.0.0 require Node >=22.18.0 while the
workflow pinned Node 20. Pre-existing mismatch (cspell ^10 predates this branch
and fails identically on main), unrelated to the discipline-guard hook changes.
Node 22 satisfies both the repo engines (>=20) and cspell (>=22.18).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.github/workflows/a11y.yml

@@ -21,10 +21,10 @@ jobs:
           extensions: pdo, pdo_pgsql, redis, mbstring, intl, bcmath
           coverage: none
 
-      - name: Setup Node 20
+      - name: Setup Node 22
         uses: actions/setup-node@v4
         with:
-          node-version: '20'
+          node-version: '22'
           cache: 'npm'
 
       - name: Install root JS deps