Add autonomous monitor: 5-min systemd timer + on-chain USDC balance check

deploy/monitor.py probes /health, /mcp/rpc initialize, /scan x402 challenge,
and reads USDC.balanceOf(wallet) on Base mainnet via public RPC. Appends one
JSON line per run to data/monitor.jsonl and persists state to detect:
- service_down / ingest_stale / mcp_transport_broken / x402_misconfig
- FIRST_PAYMENT_RECEIVED (when wallet balance crosses 0)
- balance_changed (delta vs previous run)

Installed as aegis402-monitor.{service,timer} on the VPS with hardened
systemd sandbox (NoNewPrivileges, ProtectSystem=strict, ReadWritePaths
limited to data/). Timer active, first run green: cves=500, mcp=ok,
x402 challenge returns 402 with payTo=0x3D1F...4ff6, balance=0 USDC.

deploy/aegis402-monitor.service

@@ -0,0 +1,18 @@
+[Unit]
+Description=Aegis402 monitoring probe
+After=network-online.target
+
+[Service]
+Type=oneshot
+User=aegis
+Group=aegis
+WorkingDirectory=/home/aegis/aegis402
+EnvironmentFile=/etc/aegis402.env
+ExecStart=/home/aegis/aegis402/venv/bin/python deploy/monitor.py
+StandardOutput=journal
+StandardError=journal
+NoNewPrivileges=yes
+PrivateTmp=yes
+ProtectSystem=strict
+ReadWritePaths=/home/aegis/aegis402/data
+ProtectHome=read-only