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The site applies DeployArtifactsCommand.Secrets as the full set (rows absent from it are soft-removed and DeployedSecretsMirror deletes them on both nodes), while null means 'central shipped no secrets' and touches nothing. The command builder shipped null for a site with no references, so dropping the LAST secret: reference never removed the secret from either site node. Found by the Task 13 live gate (check 5: site-a-a and site-a-b both still held sb-gate-mxgw r1, deployed_secrets.is_removed=0 after the removal deploy). Central now always ships a list. Old sites ignore proto field 9; the artifact hash is unchanged (empty and null hash identically); SecretsDeployed stays null and no DeploySiteSecrets audit row is written when nothing shipped. Regression test NoSecretReferences_ShipsEmptySecretsList_SoTheSiteRemovesPreviouslyDeployedOnes (red before, green after). Spec notes in Component-DeploymentManager/SiteRuntime + the design doc's as-built section.