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/builtin.go

@@ -18,6 +18,9 @@ type BuiltinRequest struct {
 	Stdout io.Writer
 	Stderr io.Writer
 	Info   platform.Info
+	// Confirmed reports that the user passed --yes; destructive builtins use
+	// it instead of asking for confirmation a second time.
+	Confirmed bool
 	// Plan is the resolved variant and forwarded arguments. Builtins that
 	// drive an external runtime (for example docker save) read the runtime's
 	// program name and the passthrough arguments from it.
@@ -43,6 +46,8 @@ var builtins = map[string]Builtin{
 	"docker.save":        runDockerSave,
 	"docker.stats":       runDockerStats,
 	"docker.stop":        runDockerStop,
+	"git.retime":         runGitRetime,
+	"git.retime.preview": runGitRetimePreview,
 	"java.run":           runJavaRun,
 	"java.run.preview":   runJavaPreview,
 	"net.pids":           runNetPIDs,