feat(ohos): 鸿蒙端聊天历史分段懒加载 + 首次提交完整工程

原有 cmd/ohos/HomeAgent 是未入库的鸿蒙原生 ArkTS 工程，本次随改动一并入库，
保证他人 clone 后可直接编译（含 .gitignore 排除 build/oh_modules/签名材料，
提供 build-profile.json5.example 模板）。

本次功能改动（与 WebUI / GUI 三端对齐）：
- /chat/history 首屏只拉最新 CHAT_PAGE_SIZE(40) 条，1.26MB → 48.5KB
- 抽出 parseHistoryPayload() 复用解析，记录 chatOffset/chatHasMore
- 新增 loadOlderChat()：向上滚动触顶（yOffset<60）懒加载更早页
- 工具调用 args/result 与 reasoning_content 完整还原，不做裁剪

构建验证：hvigorw assembleHap BUILD SUCCESSFUL（7.8s，ChatPage 零告警）

cmd/ohos/HomeAgent/entry/src/main/module.json5

@@ -0,0 +1,45 @@
+{
+  "module": {
+    "name": "entry",
+    "type": "entry",
+    "description": "HomeAgent client entry module",
+    "mainElement": "EntryAbility",
+    "deviceTypes": [
+      "phone",
+      "tablet"
+    ],
+    "deliveryWithInstall": true,
+    "installationFree": false,
+    "pages": "$profile:main_pages",
+    "requestPermissions": [
+      {
+        "name": "ohos.permission.INTERNET"
+      },
+      {
+        "name": "ohos.permission.GET_NETWORK_INFO"
+      }
+    ],
+    "abilities": [
+      {
+        "name": "EntryAbility",
+        "srcEntry": "./ets/entryability/EntryAbility.ets",
+        "description": "HomeAgent main entry",
+        "icon": "$media:app_icon",
+        "label": "$string:app_name",
+        "startWindowIcon": "$media:app_icon",
+        "startWindowBackground": "$color:start_window_background",
+        "exported": true,
+        "skills": [
+          {
+            "entities": [
+              "entity.system.home"
+            ],
+            "actions": [
+              "action.system.home"
+            ]
+          }
+        ]
+      }
+    ]
+  }
+}