Implement slow limpet and armid mine decay over time #435
Description
Model slow decay/removal of mines over long periods.
Parent: #430
Summary:
- Periodically reduce mine counts in sectors based on age.
- Prevents permanent mine saturation without maintenance.
Acceptance criteria:
- Cron task that decays mines according to configurable half-life.
- Sector scan output reflects updated counts.
- Decay events logged for debugging.
See epic for canon doc links.