Implements monitoring/collector.rs that collects per-container CPU/RAM/network/disk, system-wide metrics, RPC latency, and WebSocket connection count every 60 seconds. Data stored in dual ring buffers: 1-min resolution (24h) and 15-min resolution (7d). Three new RPC endpoints: monitoring.current, monitoring.history, monitoring.containers. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>