test(cli): add testscript publish e2e

[jmgilman]

Summary

• add a testscript-based imgcli publish E2E that starts imgsrv/test and a local IncusOS CDN fixture
• add internal-only CDN base URL plumbing for tests without changing the public CLI surface
• replace the older in-process publish integration test and point Moon integration testing at cmd/imgcli

Verification

• go test ./...
• go test -count 1 -tags integration ./cmd/imgcli
• moon run root:test-integration --summary minimal
• moon ci --summary minimal
• git diff --check