feat(wallet-monitor): Slack alerts for low operator wallet balances

A one-shot monitor for the vector-marketplace box: checks each wallet in
wallets.json against a threshold (default 10 APEX) via Ogmios
(ReadOnlyOgmiosProvider) and posts a combined Slack message when any wallet is
below it. Re-reminds every 6h while a wallet stays low and posts a recovery
message when it climbs back above, deduped through a persisted state file.

Runs hourly under host cron via `docker compose run --rm` — no long-running
server, no ports/healthcheck. Reuses the buyer/supplier tsx Docker pattern and
the existing balance-query client; addresses come from a static, gitignored
wallets.json so the monitor never reads a private key.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

pnpm-workspace.yaml

@@ -4,3 +4,4 @@ packages:
   - "supplier"
   - "indexer"
   - "indexer-ui"
+  - "wallet-monitor"