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.dart

@@ -34,6 +34,7 @@ enum AgUiEventType {
   unknown,
 }
 
+// wire 类型到枚举的映射
 const _wireToTypeMap = {
   AgUiEventTypeWire.runStarted: AgUiEventType.runStarted,
   AgUiEventTypeWire.runFinished: AgUiEventType.runFinished,
@@ -49,6 +50,7 @@ const _wireToTypeMap = {
   AgUiEventTypeWire.messagesSnapshot: AgUiEventType.messagesSnapshot,
 };
 
+// 枚举到 wire 类型的映射
 const _typeToWireMap = {
   AgUiEventType.runStarted: AgUiEventTypeWire.runStarted,
   AgUiEventType.runFinished: AgUiEventTypeWire.runFinished,
@@ -70,6 +72,23 @@ AgUiEventType agUiEventTypeFromWire(String wire) =>
 
 String agUiEventTypeToWire(AgUiEventType type) => _typeToWireMap[type] ?? '';
 
+// 类型到工厂函数的映射，用于简化 fromJson
+final _typeToFactory = {
+  AgUiEventType.runStarted: RunStartedEvent.fromJson,
+  AgUiEventType.runFinished: RunFinishedEvent.fromJson,
+  AgUiEventType.runError: RunErrorEvent.fromJson,
+  AgUiEventType.textMessageStart: TextMessageStartEvent.fromJson,
+  AgUiEventType.textMessageContent: TextMessageContentEvent.fromJson,
+  AgUiEventType.textMessageEnd: TextMessageEndEvent.fromJson,
+  AgUiEventType.toolCallStart: ToolCallStartEvent.fromJson,
+  AgUiEventType.toolCallArgs: ToolCallArgsEvent.fromJson,
+  AgUiEventType.toolCallEnd: ToolCallEndEvent.fromJson,
+  AgUiEventType.toolCallResult: ToolCallResultEvent.fromJson,
+  AgUiEventType.toolCallError: ToolCallErrorEvent.fromJson,
+  AgUiEventType.messagesSnapshot: MessagesSnapshotEvent.fromJson,
+  AgUiEventType.unknown: UnknownAgUiEvent.fromJson,
+};
+
 @JsonSerializable()
 class AgUiEvent {
   final AgUiEventType type;
@@ -79,35 +98,7 @@ class AgUiEvent {
   factory AgUiEvent.fromJson(Map<String, dynamic> json) {
     final typeStr = json['type'] as String? ?? '';
     final type = agUiEventTypeFromWire(typeStr);
-
-    switch (type) {
-      case AgUiEventType.runStarted:
-        return RunStartedEvent.fromJson(json);
-      case AgUiEventType.runFinished:
-        return RunFinishedEvent.fromJson(json);
-      case AgUiEventType.runError:
-        return RunErrorEvent.fromJson(json);
-      case AgUiEventType.textMessageStart:
-        return TextMessageStartEvent.fromJson(json);
-      case AgUiEventType.textMessageContent:
-        return TextMessageContentEvent.fromJson(json);
-      case AgUiEventType.textMessageEnd:
-        return TextMessageEndEvent.fromJson(json);
-      case AgUiEventType.toolCallStart:
-        return ToolCallStartEvent.fromJson(json);
-      case AgUiEventType.toolCallArgs:
-        return ToolCallArgsEvent.fromJson(json);
-      case AgUiEventType.toolCallEnd:
-        return ToolCallEndEvent.fromJson(json);
-      case AgUiEventType.toolCallResult:
-        return ToolCallResultEvent.fromJson(json);
-      case AgUiEventType.toolCallError:
-        return ToolCallErrorEvent.fromJson(json);
-      case AgUiEventType.messagesSnapshot:
-        return MessagesSnapshotEvent.fromJson(json);
-      case AgUiEventType.unknown:
-        return UnknownAgUiEvent.fromJson(json);
-    }
+    return _typeToFactory[type]?.call(json) ?? UnknownAgUiEvent.fromJson(json);
   }
 
   Map<String, dynamic> toJson() => _$AgUiEventToJson(this);
@@ -322,6 +313,7 @@ class SnapshotMessage {
   final String? content;
   final String? toolCallId;
   final UiCard? ui;
+  final DateTime? timestamp;
 
   SnapshotMessage({
     required this.id,
@@ -329,6 +321,7 @@ class SnapshotMessage {
     this.content,
     this.toolCallId,
     this.ui,
+    this.timestamp,
   });
 
   factory SnapshotMessage.fromJson(Map<String, dynamic> json) =>