add diagnostic cleanup

[wolfeaustin]

• Adds periodic cleanup of expired heartbeat/diagnostics objects from federated storage
• Configurable retention (default 7d) and cleanup interval (default 1h)
• Scoped to this agent’s cluster prefix only; delete failures are nonfatal

Notes

• Complements KCM-5386 (aggregator lookback/DB TTL); does not replace it.
• Pairs with the finops-agent-chart PR to expose Helm values. agent storage retention and diagnostic cleanup interval finops-agent-chart#115
• Time-based cleanup, not ingestion-aware. Safe under the aggregator’s default 1d lookback, but longer aggregator outages can permanently lose unrecovered source files.

Closes

• https://apptio.atlassian.net/browse/KCM-5391