feat: use cpio crate for initramfs creation

Add the `cpio` crate as dependency, removing e2fsprogs external dependency.

README.md

@@ -59,7 +59,7 @@ ecr --arch arm64 alpine -- uname -m
 # Always pull a fresh image
 ecr --no-cache fedora
 
-# Boot with QEMU system emulation (requires qemu-system-<arch> and e2fsprogs)
+# Boot with QEMU system emulation (requires qemu-system-<arch>)
 ecr --kernel /boot/vmlinuz ubuntu
 
 # Boot with custom memory
@@ -75,15 +75,14 @@ ecr --kernel /boot/vmlinuz alpine
 ```
 
 This mode:
-- Creates an ext4 disk image from the rootfs
+- Creates a gzipped CPIO initramfs from the rootfs
 - Boots QEMU with your kernel
 - Provides full VM isolation
 - Works for any architecture (no binfmt_misc needed)
 
 Requirements:
 - `qemu-system-<arch>` installed
-- `e2fsprogs` for disk image creation
-- Kernel with virtio support
+- Kernel with serial console support
 
 ## Supported distributions