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/agent.go

@@ -48,6 +48,7 @@ type Agent struct {
 	keys                      []gossh.PublicKey                                     // SSH public keys
 	smartManager              *SmartManager                                         // Manages SMART data
 	systemdManager            *systemdManager                                       // Manages systemd services
+	probeManager              *ProbeManager                                        // Manages network probes
 }
 
 // NewAgent creates a new agent with the given data directory for persisting data.
@@ -121,6 +122,9 @@ func NewAgent(dataDir ...string) (agent *Agent, err error) {
 	// initialize handler registry
 	agent.handlerRegistry = NewHandlerRegistry()
 
+	// initialize probe manager
+	agent.probeManager = newProbeManager()
+
 	// initialize disk info
 	agent.initializeDiskInfo()