harden: create rewind working-tree directories via os.Root

When entire checkpoint rewind restores files from a checkpoint tree, the file write already used os.Root (kernel-enforced containment), but the preceding directory creation used a raw os.MkdirAll(filepath.Join(repoRoot, f.Name)). A crafted tree entry name containing ".." from an untrusted checkpoint could thus create empty directories outside the repo before the guarded write refused to escape.

Add osroot.MkdirAll, which creates each level via os.Root.Mkdir so the kernel rejects any path that escapes the root, and use it in the restore loop. Both filesystem operations in that loop are now containment-safe.