docs/persistence: document the todo/reminder retention windows vacuum.rs already enforces

docs/persistence.md

@@ -326,6 +326,17 @@ Under `/var/lib/hyperhive/agents/<name>/`:
     directories; the summary is bucketed and carries no raw byte
     counts, so an unchanged situation re-upserts as `changed == false`
     and never re-wakes.
+
+    Retention, same hourly `hive-agent::vacuum::run` sweep as
+    `hyperhive-events.sqlite` below: delivered (soft-deleted) reminder
+    rows are reaped 14 days after delivery, kept that long only to serve
+    the trailing-window `ReminderRollup` stats; acked todo rows are
+    reaped 30 days after acking (long enough that only a genuinely quiet
+    month triggers the "one spurious re-announcement" fallback a
+    reconciled producer like `disk_watch` relies on — see `todos.rs`'s
+    module doc). Un-acked todos and undelivered reminders are never
+    swept — same "audit trail, not cache" treatment as the c0re-side
+    tables above.
   - `hyperhive-events.sqlite` — turn-loop event log.
   - `hyperhive-turn-stats.sqlite` — per-turn timing stats.
   - `hyperhive-model` — single-line model name override file.