[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/ai/tools/tool_registry.dart

@@ -0,0 +1,33 @@
+import 'catalog_tools.dart';
+import 'habit_tools.dart';
+import 'tool_definition.dart';
+import 'tracker_tools.dart';
+
+/// Static registry of all tools exposed to the LLM.
+///
+/// Order is the order surfaced to the model (`flutter_gemma` preserves the
+/// list). Read-only tools first, then destructive — mirrors a "look before
+/// you leap" prompt bias.
+final List<ToolDefinition> kAllTools = [
+  // read-only
+  searchCatalogTool,
+  queryProtocolTool,
+  listActiveHabitsTool,
+  getStreakTool,
+  // destructive (confirm gate)
+  addHabitTool,
+  logTrackerEntryTool,
+];
+
+class ToolRegistry {
+  final Map<String, ToolDefinition> _byName;
+
+  ToolRegistry(List<ToolDefinition> tools)
+      : _byName = {for (final t in tools) t.name: t};
+
+  factory ToolRegistry.defaults() => ToolRegistry(kAllTools);
+
+  ToolDefinition? byName(String name) => _byName[name];
+
+  Iterable<ToolDefinition> get all => _byName.values;
+}