fix(wallet-monitor): post only on state changes, drop reminder re-alerts

Slack posts now fire only when a wallet newly drops below its threshold
or recovers above it; a wallet that stays low never re-posts. The
reminder_hours mechanism is removed entirely — per-wallet reminder
clocks drifted apart (Ogmios blips skip wallets from a run), so with
many chronically-low wallets some clock came due nearly every run,
collapsing the 6h cadence to the hourly cron interval.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

wallet-monitor/wallets.example.json

@@ -1,6 +1,5 @@
 {
   "default_min_ap3x": 10,
-  "reminder_hours": 6,
   "wallets": [
     { "name": "buyer",             "address": "addr1_REPLACE_ME" },
     { "name": "supplier-ollama",   "address": "addr1_REPLACE_ME" },