feat: 新增 git retime，把指定日期范围的提交时间改到 21:00-23:00 并强推

- 保留每个提交的原日期，只重排时分秒到 21:00-23:00；同日严格递增，
  间隔 5-40 分钟且秒位随机；窗口不足时自动压紧并在预览中标注
- 支持 --window/--since/--until、--interval（含 match-remote）、--seed、
  --remote、--branch、--no-push
- 安全措施：默认只预览（退出码 2）、脏工作区拒绝、改写前创建
  refs/kopsc-backup/<分支>-<时间戳> 备份、推送前比对 tree 哈希、
  只用 --force-with-lease
- BuiltinRequest 增加 Confirmed，使内置命令能获知 CLI 已确认 --yes
- 文档：新增 README.git.md，主 README 补充导航与示例

internal/executor/gitverify_test.go

@@ -0,0 +1,57 @@
+package executor
+
+import (
+	"context"
+	"strings"
+	"testing"
+)
+
+func TestTreesMatchComparesTreeHashes(t *testing.T) {
+	runner := &fakeGitRunner{responses: map[string]gitResponse{
+		"rev-parse old^{tree}":   {out: "treeA\n"},
+		"rev-parse same^{tree}":  {out: "treeA\n"},
+		"rev-parse other^{tree}": {out: "treeB\n"},
+	}}
+	ctx := context.Background()
+
+	same, err := treesMatch(ctx, runner, "old", "same")
+	if err != nil || !same {
+		t.Fatalf("same trees: match=%v err=%v", same, err)
+	}
+	same, err = treesMatch(ctx, runner, "old", "other")
+	if err != nil || same {
+		t.Fatalf("different trees: match=%v err=%v", same, err)
+	}
+}
+
+func TestTreesMatchReportsRevParseFailure(t *testing.T) {
+	runner := &fakeGitRunner{responses: map[string]gitResponse{}}
+	_, err := treesMatch(context.Background(), runner, "missing", "other")
+	if err == nil {
+		t.Fatal("expected an error when the commit cannot be read")
+	}
+}
+
+func TestVerifyRewritePassesOnIdenticalContent(t *testing.T) {
+	runner := &fakeGitRunner{responses: map[string]gitResponse{
+		"rev-parse oldtip^{tree}": {out: "treeX\n"},
+		"rev-parse newtip^{tree}": {out: "treeX\n"},
+	}}
+	if err := verifyRewrite(context.Background(), runner, "oldtip", "newtip"); err != nil {
+		t.Fatalf("identical content should pass: %v", err)
+	}
+}
+
+func TestVerifyRewriteStopsOnContentChange(t *testing.T) {
+	runner := &fakeGitRunner{responses: map[string]gitResponse{
+		"rev-parse oldtip^{tree}": {out: "treeX\n"},
+		"rev-parse newtip^{tree}": {out: "treeY\n"},
+	}}
+	err := verifyRewrite(context.Background(), runner, "oldtip", "newtip")
+	if err == nil || !strings.Contains(err.Error(), "内容与原提交不一致") {
+		t.Fatalf("err = %v, want a content-mismatch error", err)
+	}
+	if !strings.Contains(err.Error(), "回滚") {
+		t.Errorf("error should point at the backup: %v", err)
+	}
+}