Add ModelPack created annotation to manifests

[gnanirahulnutakki]

Closes #1138

This adds org.opencontainers.image.created to ModelPack manifests only, and it reuses the same CreatedAt value already emitted into the ModelPack config instead of minting a second timestamp at manifest creation time. That keeps the manifest aligned with the existing ModelPack metadata while leaving KitFormat behavior unchanged.

Validation:

• go test ./pkg/lib/filesystem -run TestCreateManifest
• go build ./...
• go generate ./... (completed locally; the harness UI generator emitted pnpm warnings in this environment but produced no tracked source changes)

[gnanirahulnutakki]

Pushed a follow-up on April 30, 2026 to fix the failing Check sources job on this branch.

The failure was mechanical: the new Go test file was missing the required Apache license header. Current local recheck on the updated branch:

/opt/homebrew/bin/go test ./pkg/lib/filesystem
$HOME/go/bin/addlicense --check -s -l apache -c "The KitOps Authors." pkg/lib/filesystem/local-storage_test.go

Both pass locally.

The new GitHub workflow runs on the fork are currently showing action_required with no jobs started yet on my side, so it looks like they may need maintainer approval before the updated PR jobs execute.