[codex] add platform flag for images and runs

[rgarcia]

Summary

Adds a --platform flag to the Hypeman CLI for image creation/pull and instance creation.

This lets local macOS users exercise the Rosetta support from hypeman PR 279 directly through the CLI, for example:

hypeman run --platform linux/amd64 --hypervisor vz docker.io/library/alpine:3.19

Details

• Adds --platform to hypeman image create and hypeman pull.
• Adds --platform to hypeman run.
• Threads the platform through image auto-pull during run when the image is missing.
• Threads the platform through the final instance create request.
• Uses the SDK's option.WithJSONSet because the currently generated hypeman-go version does not yet expose typed platform fields.

Server-side instance creation is responsible for resolving/validating the requested platform when an image tag already exists locally.

Validation

go test ./pkg/cmd ./cmd/hypeman
go build -o bin/hypeman ./cmd/hypeman
./bin/hypeman run --help | rg -n "platform|amd64"
./bin/hypeman pull --help | rg -n "platform"
./bin/hypeman image create --help | rg -n "platform"