ci: bootstrap build/lint/test workflow and wire eslint

.github/workflows/ci.yml

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+name: CI
+
+# Job ids are build / lint / test on purpose: they become the status check
+# contexts, and the protect-main ruleset requires those exact names. Renaming a
+# job here silently makes its required check unsatisfiable.
+
+on:
+  pull_request:
+  push:
+    branches: [main]
+
+permissions:
+  contents: read
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    timeout-minutes: 10
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 22
+          cache: npm
+      - run: npm ci
+      - run: npm run build
+
+  lint:
+    runs-on: ubuntu-latest
+    timeout-minutes: 10
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 22
+          cache: npm
+      - run: npm ci
+      - run: npm run lint
+
+  test:
+    runs-on: ubuntu-latest
+    timeout-minutes: 10
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 22
+          cache: npm
+      - run: npm ci
+      - run: npm test

.prettierrc

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+{
+  "singleQuote": true,
+  "trailingComma": "all"
+}

eslint.config.js

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+// Flat config. ESLint 9 ignores .eslintrc.*, which is why the previous
+// .eslintrc.js never actually ran in this repo.
+const universeNative = require('eslint-config-universe/flat/native');
+const universeWeb = require('eslint-config-universe/flat/web');
+
+module.exports = [
+  {
+    ignores: ['build/**', 'plugin/build/**', 'coverage/**', 'node_modules/**'],
+  },
+  ...universeNative,
+  ...universeWeb,
+];