修复: 文件日志 + G1 日志增强 + 清理死代码

- 文件日志: tracing-subscriber + tracing-appender 写入 %TEMP%/devflow-trace.log
- G1 日志: 增加 first_goal 字段显示首条目标内容
- 清理: 删 workspace_root_str / data_dir_path 死代码
- 标注: default_with_root 加 cfg_attr 抑制 non-test 编译警告

Cargo.lock

@@ -859,6 +859,8 @@ dependencies = [
  "tauri-plugin-window-state",
  "tokio",
  "tracing",
+ "tracing-appender",
+ "tracing-subscriber",
  "tree-sitter",
  "tree-sitter-go",
  "tree-sitter-java",
@@ -4201,6 +4203,12 @@ dependencies = [
  "serde_json",
 ]
 
+[[package]]
+name = "symlink"
+version = "0.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a7973cce6668464ea31f176d85b13c7ab3bba2cb3b77a2ed26abd7801688010a"
+
 [[package]]
 name = "syn"
 version = "1.0.109"
@@ -5039,6 +5047,19 @@ dependencies = [
  "tracing-core",
 ]
 
+[[package]]
+name = "tracing-appender"
+version = "0.2.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "050686193eb999b4bb3bc2acfa891a13da00f79734704c4b8b4ef1a10b368a3c"
+dependencies = [
+ "crossbeam-channel",
+ "symlink",
+ "thiserror 2.0.18",
+ "time",
+ "tracing-subscriber",
+]
+
 [[package]]
 name = "tracing-attributes"
 version = "0.1.31"