refactor(beaver): 移除Hermes相关引用和迁移代码，完善Beaver后端主线实现

移除了所有Hermes相关的命名引用，包括：
- 从.gitignore中清理相关构建缓存文件
- 将README中的beaver-home路径配置更新
- 完善backend/README.md文档说明Beaver后端主线实现
- 移除Hermes风格的相关注释和兼容性代码
- 清理nanobot环境变量兼容性处理
- 删除技能迁移和服务迁移相关功能代码
- 更新测试用例中相关命名和函数名

BREAKING CHANGE: 移除了Hermes迁移相关API和CLI命令，不再支持nanobot环境变量兼容性

app-instance/backend/beaver/services/cron_service.py

@@ -38,10 +38,9 @@ _MAX_HISTORY = 20
 class CronService:
     """Persistent single-timer scheduler.
 
-    Hermes' cron implementation stores jobs as JSON and ticks safely in the
-    background. Beaver keeps that shape, but the callback is required to route
-    agent work through Task mode so every scheduled trigger is visible as a
-    normal Task.
+    Jobs are stored as JSON and ticked safely in the background. The callback
+    routes agent work through Task mode so every scheduled trigger is visible as
+    a normal Task.
     """
 
     def __init__(self, store_path: str | Path, *, on_job: CronCallback | None = None) -> None:

app-instance/backend/beaver/services/memory_service.py

@@ -43,7 +43,7 @@ class MemoryService:
     def reload_for_new_run(self) -> None:
         """每次新 run 开始前刷新 live state。
 
-        这是 Hermes 风格 memory policy 的关键点：
+        这是 Beaver memory policy 的关键点：
         - 上一次会话中通过 tool 写入的持久记忆，下一次运行应该能看到
         - 但同一次 run 中途写入的新记忆，不应反向修改当前 frozen snapshot
         """