新增: 初始化 DevFlow 项目仓库

Tauri 2 + Vue 3 + Vite 6 桌面应用，Rust workspace 含 13 个 crate
(df-ai / df-storage / df-workflow / df-core / df-execute 等)。
核心能力：AI 聊天 agentic 循环(工具调用+人工审批)、工作流引擎、
任务/想法/项目/阶段管理、可追溯性，及配套前端组件。

crates/df-workflow/Cargo.toml

@@ -0,0 +1,15 @@
+[package]
+name = "df-workflow"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+df-core = { path = "../df-core" }
+serde = { workspace = true }
+serde_json = { workspace = true }
+tokio = { workspace = true }
+async-trait = { workspace = true }
+futures = "0.3"
+thiserror = { workspace = true }
+anyhow = { workspace = true }
+tracing = { workspace = true }