ci: add Gitea Actions workflow

Runs npm ci, typecheck, build, test, and a 'git diff --exit-code dist/'
gate that catches 'forgot to re-run npm run build' before merge. Fires on
PR / push to main / push to v* tags.

.gitea/workflows/ci.yml

@@ -0,0 +1,21 @@
+name: ci
+on:
+  push:
+    branches: [main]
+    tags: ['v*']
+  pull_request:
+    branches: [main]
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-node@v4
+        with:
+          node-version: '20'
+      - run: npm ci
+      - run: npm run typecheck
+      - run: npm run build
+      - run: npm test
+      - name: ensure dist is up to date
+        run: git diff --exit-code dist/