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

src-tauri/Cargo.toml

@@ -33,6 +33,7 @@ df-execute = { path = "../crates/df-execute" }
 df-ai = { path = "../crates/df-ai" }
 df-ideas = { path = "../crates/df-ideas" }
 df-project = { path = "../crates/df-project" }
+df-mcp = { path = "../crates/df-mcp" }
 futures = "0.3"
 # write_file base64 编码:写二进制/非 UTF-8 文件(图片/PDF/Excel 等)
 base64 = "0.22"