Fix image pull unpacking all platforms by default

[CHE3MZ]

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Motivation and Context

container image pull without --platform unpacks a 512 GiB sparse snapshot per architecture in the manifest (related: #784), while container run unpacks only the requested platform via getCreateSnapshot. For a multi-arch image like alpine:latest this means roughly 3 GB allocated after pull versus 1 GB after run for the same entry in image list.

Fetch behavior is unchanged (all blobs are still downloaded, preserving offline use); only the unpack step defaults to the host platform. --all-platforms restores the previous unpack-all behavior, and other platforms are still unpacked on demand by run --platform.

Testing

• Tested locally (build: make container reaches Build complete on macOS arm64; functional pull-size comparison not yet run)
• Added/updated tests (no existing unit coverage for ImagePull)
• Added/updated docs (command-reference.md: --all-platforms usage and option)