refactor(chat): 重构聊天模块并集成历史消息加载功能

- 删除冗余的 chat_history_repository 和 home_mock_data
- 简化 ag_ui_event fromJson 使用工厂映射表
- 提取 ChatBloc 事件处理方法，添加 loadHistory/loadMoreHistory
- HomeScreen 集成 ChatBloc 实现历史消息加载和下拉刷新
- 更新 AGENTS.md 文档约束

apps/lib/features/chat/data/models/ag_ui_event.g.dart

@@ -25,6 +25,7 @@ const _$AgUiEventTypeEnumMap = {
   AgUiEventType.toolCallEnd: 'toolCallEnd',
   AgUiEventType.toolCallResult: 'toolCallResult',
   AgUiEventType.toolCallError: 'toolCallError',
+  AgUiEventType.messagesSnapshot: 'messagesSnapshot',
   AgUiEventType.unknown: 'unknown',
 };
 
@@ -157,3 +158,39 @@ Map<String, dynamic> _$ToolCallErrorEventToJson(ToolCallErrorEvent instance) =>
       'error': instance.error,
       'code': instance.code,
     };
+
+MessagesSnapshotEvent _$MessagesSnapshotEventFromJson(
+  Map<String, dynamic> json,
+) => MessagesSnapshotEvent(
+  messages: (json['messages'] as List<dynamic>)
+      .map((e) => SnapshotMessage.fromJson(e as Map<String, dynamic>))
+      .toList(),
+);
+
+Map<String, dynamic> _$MessagesSnapshotEventToJson(
+  MessagesSnapshotEvent instance,
+) => <String, dynamic>{'messages': instance.messages};
+
+SnapshotMessage _$SnapshotMessageFromJson(Map<String, dynamic> json) =>
+    SnapshotMessage(
+      id: json['id'] as String,
+      role: json['role'] as String,
+      content: json['content'] as String?,
+      toolCallId: json['toolCallId'] as String?,
+      ui: json['ui'] == null
+          ? null
+          : UiCard.fromJson(json['ui'] as Map<String, dynamic>),
+      timestamp: json['timestamp'] == null
+          ? null
+          : DateTime.parse(json['timestamp'] as String),
+    );
+
+Map<String, dynamic> _$SnapshotMessageToJson(SnapshotMessage instance) =>
+    <String, dynamic>{
+      'id': instance.id,
+      'role': instance.role,
+      'content': instance.content,
+      'toolCallId': instance.toolCallId,
+      'ui': instance.ui,
+      'timestamp': instance.timestamp?.toIso8601String(),
+    };