Wire design_system into CI, and drop EmptyState's speculative action slot

The pre-push coverage gate only ran for whoever had the local hook
installed, which is deployment hygiene rather than a gate. Adds the
mirrored workflow sync_settings_ui already has, so the 100% bar is
enforced on push and PR.

EmptyState.action was invented surface: no donor had it. home_inventory's
three call sites are pure placeholders, and kuhylog's action-bearing
_empty was explicitly excluded from the survey as a different element.
Shipping a parameter nobody asked for on a widget that has no consumer
yet is the failure mode this package is supposed to avoid, so it goes,
with a comment recording why rather than leaving the next reader to
rediscover it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XS1m7b1piBtAL22USLW6jL

.github/workflows/design-system-tests.yml

@@ -0,0 +1,36 @@
+name: design_system tests
+
+on:
+  push:
+    branches: [main]
+    paths: ['design_system/**']
+  pull_request:
+    branches: [main]
+    paths: ['design_system/**']
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        working-directory: design_system
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up Flutter
+        uses: subosito/flutter-action@v2
+        with:
+          channel: stable
+
+      - name: Install dependencies
+        run: flutter pub get
+
+      - name: Analyze
+        run: flutter analyze
+
+      - name: Run tests with coverage
+        run: flutter test --coverage
+
+      - name: Enforce 100% line coverage
+        run: scripts/check_coverage.sh coverage/lcov.info 100

design_system/lib/src/empty_state.dart

@@ -14,7 +14,6 @@ class EmptyState extends StatelessWidget {
     required this.icon,
     required this.title,
     required this.message,
-    this.action,
     super.key,
   });
 
@@ -27,15 +26,14 @@ class EmptyState extends StatelessWidget {
   /// One line explaining what to do about it.
   final String message;
 
-  /// Optional call to action below the message — usually the button that
-  /// creates the first item. Omit it where the empty state is a *filter*
-  /// result rather than an empty collection: there is nothing to create.
-  final Widget? action;
+  // Deliberately no `action` slot. No donor had one: home_inventory's three
+  // call sites are pure placeholders, and kuhylog's action-bearing `_empty`
+  // is a different element. Adding one here would be speculative surface --
+  // the widget nobody imports, one parameter down.
 
   @override
   Widget build(BuildContext context) {
     final theme = Theme.of(context);
-    final action = this.action;
     return Center(
       child: Padding(
         padding: const EdgeInsets.all(AppSpacing.xl),
@@ -57,10 +55,6 @@ class EmptyState extends StatelessWidget {
                 color: theme.colorScheme.onSurfaceVariant,
               ),
             ),
-            if (action != null) ...[
-              const SizedBox(height: AppSpacing.lg),
-              action,
-            ],
           ],
         ),
       ),

design_system/test/empty_state_test.dart

@@ -2,16 +2,15 @@ import 'package:design_system/design_system.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter_test/flutter_test.dart';
 
-Future<void> _pumpEmptyState(WidgetTester tester, {Widget? action}) {
+Future<void> _pumpEmptyState(WidgetTester tester) {
   return tester.pumpWidget(
     MaterialApp(
       theme: buildDarkTheme(),
-      home: Scaffold(
+      home: const Scaffold(
         body: EmptyState(
           icon: Icons.inbox_outlined,
           title: 'Nothing here yet',
           message: 'Add your first item to get started.',
-          action: action,
         ),
       ),
     ),
@@ -49,17 +48,11 @@ void main() {
     expect(title.style?.color, isNot(scheme.onSurfaceVariant));
   });
 
-  testWidgets('omits the action slot by default', (tester) async {
+  testWidgets('is a pure placeholder with no action', (tester) async {
+    // Deliberate: no donor had an action slot, so adding one would be
+    // speculative surface on a widget that has no consumer yet.
     await _pumpEmptyState(tester);
-    expect(find.byType(ElevatedButton), findsNothing);
-  });
-
-  testWidgets('renders an action when one is given', (tester) async {
-    await _pumpEmptyState(
-      tester,
-      action: ElevatedButton(onPressed: () {}, child: const Text('Add item')),
-    );
-    expect(find.text('Add item'), findsOneWidget);
+    expect(find.byType(ButtonStyleButton), findsNothing);
   });
 
   testWidgets('stays centred and shrink-wrapped', (tester) async {