New hidden subcommand that turns a `docker export`-style rootfs tar
into a banger bundle (`rootfs.ext4` + `manifest.json`, tar+zstd):
1. FlattenTar (new in imagepull) extracts the stream into a staging
dir while capturing per-file uid/gid/mode into a Metadata record.
2. imagepull.BuildExt4 produces the ext4 via `mkfs.ext4 -d`.
3. imagepull.ApplyOwnership re-applies the captured metadata with
`debugfs sif` so setuid/root-owned files keep their identity.
4. imagepull.InjectGuestAgents drops the vsock agent + network
bootstrap + first-boot service into the ext4.
5. manifest.json is written with name/distro/arch/kernel_ref.
6. Both files are packaged as .tar.zst with max compression.
Flags: --rootfs-tar (file or '-' for stdin), --name, --distro, --arch,
--kernel-ref, --description, --size, --out. Stdout prints bundle path,
sha256, and size so callers can patch the catalog.
Unit tests cover flag registration, required-arg validation, the
bundle tar round-trip, sha256HexFile, and dirSize. An end-to-end test
runs the full pipeline against a synthesized tiny rootfs tar; skips
gracefully when mkfs.ext4 / debugfs / companion binaries are missing.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
