chore: add AGENTS.md, helper scripts, codex templates, and CI

.github/workflows/flutter_ci.yml

@@ -0,0 +1,31 @@
+name: Flutter CI
+
+on:
+  pull_request:
+  push:
+    branches: [ main, master ]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Setup Flutter
+        uses: subosito/flutter-action@v2
+        with:
+          channel: stable
+
+      - name: Flutter Pub Get
+        run: flutter pub get
+
+      - name: Format check
+        run: dart format --output=none --set-exit-if-changed .
+
+      - name: Analyze
+        run: flutter analyze
+
+      - name: Test
+        run: flutter test
+