新增: MCP Server(df-mcp crate + stdio 协议 + CLI 子命令)

依据 F-260619-02 设计,对外暴露数据层接入 Claude Code/Cursor:
- 新建 df-mcp crate:JSON-RPC 2.0 over stdio(initialize/tools/list/tools/call)
- 暴露 19 数据层工具(项目/任务/灵感/工作流/回收站),复用 df-storage Repo 零重复 CRUD
- 三层安全降级:High 风险(delete/purge/run_workflow)tools/list 不暴露+dispatch 拒+handler 二次拒;Medium 允许+审计日志;--read-only 仅 Low
- 不暴露文件系统工具(防绕过应用内路径校验)
- Tauri CLI 子命令 devflow mcp-server [--db/--read-only/--print-config]
- SQLite WAL 已启用(复用 Database::open),应用与 MCP server 并发写安全

自验: df-mcp 8 passed + workspace EXIT 0

crates/df-mcp/Cargo.toml

@@ -0,0 +1,19 @@
+[package]
+name = "df-mcp"
+version = "0.1.0"
+edition = "2021"
+description = "DevFlow MCP Server — 对外暴露数据层工具(JSON-RPC 2.0 over stdio)"
+
+[dependencies]
+df-storage = { path = "../df-storage" }
+df-types = { path = "../df-types" }
+serde.workspace = true
+serde_json.workspace = true
+tokio.workspace = true
+anyhow.workspace = true
+tracing.workspace = true
+uuid.workspace = true
+futures = "0.3"
+
+[dev-dependencies]
+tokio = { workspace = true, features = ["full", "test-util"] }