[Bug]: `builder.workspace-store.fs-spec` config property documented but never implemented

## Bug Report

### 现象
配置 `builder.workspace-store.fs-spec=local`（或环境变量 `BUILDER_WORKSPACE_FS_SPEC=local`）后启动，应用仍然报错：
```
filesystem(RemoteFilesystemSpec) is designed for distributed / multi-replica deployments,
but the effective AgentStateStore is a local in-process implementation (InMemoryAgentStateStore).
```

### 根因
`BuilderConfig.java` 的 `builderBootstrap()` 方法中，`configureAllAgents` lambda 硬编码了 `RemoteFilesystemSpec`：

agentscope-examples/agents/agentscope-builder/src/main/java/io/agentscope/builder/web/config/BuilderConfig.java#L220-L224

```java
b.filesystem(
new RemoteFilesystemSpec(baseStore)
.isolationScope(IsolationScope.USER)
.addSharedPrefix("activity/"));
```

而配置属性 `builder.workspace-store.fs-spec` **在任何 Java 源码中均未被读取**（全仓搜索零结果，仅在 README 中有文档）。

### 预期行为
`builder.workspace-store.fs-spec` 应被读取并按值切换：
- `local` → `new LocalFilesystemSpec()`
- `sandbox` / `remote` → `new RemoteFilesystemSpec(baseStore)`

### 修复方向
在 `BuilderConfig.java` 中注入该属性，按值分支选择 `LocalFilesystemSpec` 或 `RemoteFilesystemSpec`。

### 受影响版本
`agentscope-builder-2.0.0-SNAPSHOT`