feat: add scion images build command

[fijimunkii]

Summary

• Adds scion images build command that wraps image-build/scripts/build-images.sh, so users can build container images directly from the CLI without needing to find and invoke the script manually
• Supports all the same flags as the script: --registry (required), --target, --push, --platform, --tag, plus --source to specify the repo clone path when not running from within it
• Exempts the images command group from grove and registry requirements in PersistentPreRunE — it's used to set up the registry before those things exist

Test plan

• Unit tests added covering: command/flag registration, missing script error, correct arg passthrough with and without optional flags
• Manually verified end-to-end: scion images build correctly invokes docker buildx with expected arguments
• Pre-existing test failures in pkg/agent and pkg/runtimebroker confirmed unrelated to this change

Note: Depends on fix for build-images.sh empty platform array bug to work without --platform.

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