feat(incusos): wire build e2e image injection

[jmgilman]

Summary

• Wire imgcli build through IncusOS CDN download, seed archive generation, and image injection.
• Return real build artifact metadata and print the final artifact path from the CLI.
• Add offline provider and CLI tests for orchestration, output path derivation, dependency errors, and collaborator error propagation.

Verification

• go test ./...
• moon run root:check
• Manual smoke with IncusOS testing version 202604261712 produced a 586 MiB raw.gz artifact.
• Verified the smoke artifact contains install.yaml at decompressed byte offset 2148532224.