feat(01-04): wire Cobra CLI with capture-classify-aggregate pipeline

- cmd/netsynth/main.go: Cobra root command with -i, --list-interfaces, --verbose flags
- Signal handling via signal.NotifyContext for clean Ctrl+C shutdown
- 3-stage pipeline: capture.StartCapture -> classify.NewClassifier -> aggregate.Aggregate
- Permission errors surface platform-specific sudo/setcap hint (CAPT-04)
- --verbose flag wires aggregate.PrintWindowLine callback for per-window output
- Exit summary via aggregate.PrintSummary with dropped packet warning
- cmd/netsynth/main_test.go: 3 tests covering flag parsing and help output
- go.mod: cobra v1.10.2 and go-pcap promoted to direct dependencies

go.mod

@@ -2,11 +2,16 @@ module github.com/netsynth/netsynth
 
 go 1.24.1
 
-require github.com/gopacket/gopacket v1.5.0
+require (
+	github.com/gopacket/gopacket v1.5.0
+	github.com/packetcap/go-pcap v0.0.0-20251215121130-f2cf9f991e7c
+	github.com/spf13/cobra v1.10.2
+)
 
 require (
-	github.com/packetcap/go-pcap v0.0.0-20251215121130-f2cf9f991e7c // indirect
+	github.com/inconshreveable/mousetrap v1.1.0 // indirect
 	github.com/sirupsen/logrus v1.9.3 // indirect
+	github.com/spf13/pflag v1.0.9 // indirect
 	golang.org/x/net v0.39.0 // indirect
 	golang.org/x/sys v0.32.0 // indirect
 )