feat(cli): implement plan command

[jmgilman]

Summary

• implement imgcli plan CONFIG with resolved-plan JSON output
• refactor IncusOS artifact planning behind Provider.Plan and share it with build
• add provider and CLI coverage for plan output and planning failures

Testing

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