Function prune_artifact_cache 

pub fn prune_artifact_cache(
    cache_root: &Path,
    namespace: &str,
    policy: ArtifactCachePrunePolicy,
) -> Result<ArtifactCachePruneReport, ArtifactCacheError>

Prune one transactional artifact namespace without acquiring an artifact.

This strict maintenance entry point returns cache errors directly. Build acquisitions configured with ArtifactCacheSpec::with_prune_policy use the same implementation but attach maintenance as a nonfatal record. Abandoned staging is removed only when its content-key lock is currently unowned and is reported separately from committed-entry retention.