refactor(go)!: 分离练习区与独立项目区

- 将现有练习及 module 文件迁移至 go/practice，保持代码和依赖不变
- 新增 go/projects 项目区说明，更新目录约定与开发命令

BREAKING CHANGE: Go 练习命令需改在 go/practice 目录下执行

go/practice/testing/slice/slice_test.go

@@ -0,0 +1,18 @@
+package main
+
+import (
+	"fmt"
+	"testing"
+)
+
+func appendSlice(s []int, n int) {
+	s = append(s, n)
+	fmt.Printf("len=%d,cap=%d\n", len(s), cap(s))
+}
+func TestSliceAppend(t *testing.T) {
+	s1 := make([]int, 5)
+	appendSlice(s1, 10)
+	if len(s1) != 5 || cap(s1) != 5 {
+		t.Errorf("Expected len=5, cap=5; got len=%d, cap=%d", len(s1), cap(s1))
+	}
+}