增强: 任务查询支持按状态过滤(list_tasks status)

- AI 工具 list_tasks: schema 加 status 参数 + handler 内存过滤
  (todo/in_progress/in_review/testing/blocked/done/cancelled)
- df-mcp list_tasks: handler 加 status 过滤(MCP 对外一致)
  (spec 声明待补:中文 Edit 编码匹配限制,后续补)

自验: workspace EXIT 0 + df-mcp cargo check EXIT 0

crates/df-mcp/src/tools.rs

@@ -346,12 +346,16 @@ fn bind_directory(ctx: &Ctx, args: Value) -> BoxFuture<'static, CallToolResult>
 fn list_tasks(ctx: &Ctx, args: Value) -> BoxFuture<'static, CallToolResult> {
     let db = ctx.db.clone();
     let project_id_filter = args.get("project_id").and_then(|v| v.as_str()).map(|s| s.to_owned());
+    let status_filter = args.get("status").and_then(|v| v.as_str()).map(|s| s.to_owned());
     Box::pin(async move {
         let repo = TaskRepo::new(&db);
         match repo.list_active().await {
             Ok(mut list) => {
-                if let Some(pid) = project_id_filter {
-                    list.retain(|t| t.project_id == pid);
+                if let Some(pid) = &project_id_filter {
+                    list.retain(|t| t.project_id == *pid);
+                }
+                if let Some(st) = &status_filter {
+                    list.retain(|t| t.status == *st);
                 }
                 json_ok(json!({ "tasks": list, "count": list.len() }))
             }