feat(agent): add ProbeManager with ICMP/TCP/HTTP probes and handlers

Implements the core probe execution engine (ProbeManager) that runs
network probes on configurable intervals, collects latency samples,
and aggregates results over a 60s sliding window. Adds two new
WebSocket handlers (SyncNetworkProbes, GetNetworkProbeResults) for
hub-agent communication and integrates probe lifecycle into the agent.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

agent/connection_manager.go

@@ -112,6 +112,7 @@ func (c *ConnectionManager) Start(serverOptions ServerOptions) error {
 		case <-sigCtx.Done():
 			slog.Info("Shutting down", "cause", context.Cause(sigCtx))
 			_ = c.agent.StopServer()
+			c.agent.probeManager.Stop()
 			c.closeWebSocket()
 			return health.CleanUp()
 		}