Clean up CI workflow

.github/workflows/main.yml

@@ -1,26 +1,16 @@
 name: Main
 
-on: [pull_request, push]
+on:
+  pull_request:
+  push:
+    branches: main
 
 jobs:
-  build:
+  test:
     runs-on: ubuntu-latest
     steps:
-      # https://github.com/actions/checkout
-      - name: Checkout
-        uses: actions/checkout@v4
-      - name: Install
-        run: npm ci
-      - name: Build
-        run: npm run build
-      - name: Test
-        run: npm run test
-      - name: Format
-        run: npm run fmtcheck
-      # - name: "check for uncommitted changes"
-      #   # Ensure no changes, but ignore node_modules dir since dev/fresh ci deps installed.
-      #   run: |
-      #     git diff --exit-code --stat -- . ':!node_modules' \
-      #     || (echo "##[error] found changed files after build. please 'npm run build && npm run fmt'" \
-      #             "and check in all changes" \
-      #         && exit 1)
+      - uses: actions/checkout@v4
+      - run: npm ci
+      - run: npm run build
+      - run: npm run test
+      - run: npm run fmtcheck