feat: Wire memory hygiene retention policy into heartbeat loop (#629)

* feat: Wire memory hygiene retention policy into heartbeat loop

* review fix

* linter fix

* fix tests

src/agent/heartbeat.rs

@@ -164,6 +164,7 @@ impl HeartbeatRunner {
                 if report.had_work() {
                     tracing::info!(
                         daily_logs_deleted = report.daily_logs_deleted,
+                        conversation_docs_deleted = report.conversation_docs_deleted,
                         "heartbeat: memory hygiene deleted stale documents"
                     );
                 }