feat(cli): publish IncusOS releases to imgsrv

[jmgilman]

Summary

• publish now builds IncusOS artifacts, uploads them to imgsrv CAS, creates/publishes catalog versions, and applies aliases
• adds release manifest JSON output plus required imgsrv URL/token/release-version validation
• adds release publisher orchestration, catalog mocks, and imgsrv harness integration coverage

Validation

• go test -count=1 ./...
• go test -count=1 -tags=integration ./internal/integration
• moon ci --summary minimal
• git diff --check