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

src-tauri/Cargo.toml

@@ -22,6 +22,8 @@ serde_json.workspace = true
 tokio.workspace = true
 anyhow.workspace = true
 tracing.workspace = true
+tracing-subscriber = { version = "0.3", features = ["env-filter"] }
+tracing-appender = "0.2"
 chrono.workspace = true
 # augmentation::MentionResolver async trait(Input Augmentation 层核心设计2)
 async-trait = { workspace = true }