fix(tool): 相对路径文件写入失败 — PathValidator 先基于 workspace 解析

当 file_write 收到相对路径如 test_tool.txt 时，PathValidator 的
resolve_and_validate 尝试对空父目录 canonicalize 导致失败。

修复：相对路径先基于 workspace_root 解析为绝对路径，再进行安全校验。

crates/zclaw-runtime/src/tool/builtin/path_validator.rs

@@ -230,7 +230,14 @@ impl PathValidator {
     fn resolve_and_validate(&self, path: &str) -> Result<PathBuf> {
         // Expand tilde
         let expanded = expand_tilde(path);
-        let path_buf = PathBuf::from(&expanded);
+        let mut path_buf = PathBuf::from(&expanded);
+
+        // If relative path and workspace is configured, resolve against workspace
+        if path_buf.is_relative() {
+            if let Some(ref workspace) = self.workspace_root {
+                path_buf = workspace.join(&path_buf);
+            }
+        }
 
         // Check for path traversal
         self.check_path_traversal(&path_buf)?;