Extract independent MIT Mortar kit and contribution workflow

.gitea/workflows/check.yml

@@ -0,0 +1,22 @@
+name: Mortar contribution checks
+on:
+  push:
+    branches: [main, "codex/**"]
+  pull_request:
+    branches: [main]
+jobs:
+  quality:
+    name: Mortar baseline and browser checks
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          persist-credentials: false
+      - uses: actions/setup-node@v4
+        with:
+          node-version: "22"
+      - run: npm ci --ignore-scripts
+      - run: npm run check
+      - run: npx playwright install --with-deps chromium
+      - run: npm run test:browser
+# No package-publishing or cloud-governance credentials belong in this workflow.