[03-Developer] #260 in-app tool calling (Gemma 4 multi-turn)

ADR-0005 in-process tool runtime — 6 tools (catalog 2 + tracker 2 +
habit 2), ToolDispatcher with JSON-schema validation + modal ConfirmGate
for destructive ops, multi-turn LlmChatSession abstraction wired to
flutter_gemma 0.16.5 (ToolChoice.auto), ChatSessionController with
MAX_TURNS=4 safety + 8-turn history hint, ChatScreen entry behind AI
opt-in. R3/R7/R8 enforced inside handlers. 41 new tests (envelope,
catalog/tracker/habit tools, dispatcher, controller loop) — 151 total
passing.

Refs #260

app/lib/ui/screens/habit_list_screen.dart

@@ -1,7 +1,9 @@
 import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 
+import '../../state/ai_providers.dart';
 import '../../state/providers.dart';
+import 'chat_screen.dart';
 import 'check_in_screen.dart';
 import 'habit_create_screen.dart';
 import 'protocol_gallery_screen.dart';
@@ -15,11 +17,22 @@ class HabitListScreen extends ConsumerWidget {
   Widget build(BuildContext context, WidgetRef ref) {
     final boot = ref.watch(bootstrapProvider);
     final habitsAsync = ref.watch(activeHabitsProvider);
+    final aiOptIn = ref.watch(aiSettingsProvider).valueOrNull ?? false;
 
     return Scaffold(
       appBar: AppBar(
         title: const Text('습관'),
         actions: [
+          if (aiOptIn)
+            IconButton(
+              icon: const Icon(Icons.smart_toy_outlined),
+              tooltip: 'AI 코치',
+              onPressed: () {
+                Navigator.of(context).push(MaterialPageRoute(
+                  builder: (_) => const ChatScreen(),
+                ));
+              },
+            ),
           IconButton(
             icon: const Icon(Icons.search),
             tooltip: '카탈로그 탐색',