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/probe_ping.go

@@ -0,0 +1,44 @@
+package agent
+
+import (
+	"os/exec"
+	"regexp"
+	"runtime"
+	"strconv"
+	"time"
+)
+
+var pingTimeRegex = regexp.MustCompile(`time[=<]([\d.]+)\s*ms`)
+
+// probeICMP executes system ping command and parses latency. Returns -1 on failure.
+func probeICMP(target string) float64 {
+	var cmd *exec.Cmd
+	switch runtime.GOOS {
+	case "windows":
+		cmd = exec.Command("ping", "-n", "1", "-w", "3000", target)
+	default: // linux, darwin, freebsd
+		cmd = exec.Command("ping", "-c", "1", "-W", "3", target)
+	}
+
+	start := time.Now()
+	output, err := cmd.Output()
+	if err != nil {
+		// If ping fails but we got output, still try to parse
+		if len(output) == 0 {
+			return -1
+		}
+	}
+
+	matches := pingTimeRegex.FindSubmatch(output)
+	if len(matches) >= 2 {
+		if ms, err := strconv.ParseFloat(string(matches[1]), 64); err == nil {
+			return ms
+		}
+	}
+
+	// Fallback: use wall clock time if ping succeeded but parsing failed
+	if err == nil {
+		return float64(time.Since(start).Microseconds()) / 1000.0
+	}
+	return -1
+}