feat(cli): add build artifact metadata output

[jmgilman]

Summary

• add imgcli build --format table|json|paths, with table as the default and paths preserving the previous output
• write per-artifact <artifact>.artifact.json sidecars using the generated ArtifactMetadata schema
• cover build output formats, deterministic multi-variant output, sidecar content, digest handling, and file permissions

Validation

• moon run root:schemas-generate
• proto run go -- test -count=1 ./internal/cli
• go test -count=1 ./internal/providers/incusos ./internal/publish ./internal/cli
• git diff --check
• moon run root:check --summary minimal