feat: add human detection scoring and webhook notifications

Implement phase 2.1 (human detection) and 2.2 (notifications):

- Detection scorer computes 0.0-1.0 human likelihood from keystroke
  timing variance, special key usage, typing speed, command diversity,
  and session duration
- Webhook notifier sends JSON POST to configured endpoints with
  deduplication, custom headers, and event filtering
- RecordingChannel gains an event callback for feeding keystrokes
  to the scorer without coupling shell and detection packages
- Server wires scorer into session lifecycle with periodic updates
  and threshold-based notification triggers
- Web UI shows human score in session tables with highlighting
- New config sections: [detection] and [[notify.webhooks]]

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/web/templates.go

@@ -2,6 +2,7 @@ package web
 
 import (
 	"embed"
+	"fmt"
 	"html/template"
 	"time"
 )
@@ -26,6 +27,18 @@ func loadTemplates() (*template.Template, error) {
 			}
 			return *t
 		},
+		"derefFloat": func(f *float64) float64 {
+			if f == nil {
+				return 0
+			}
+			return *f
+		},
+		"formatScore": func(f *float64) string {
+			if f == nil {
+				return "-"
+			}
+			return fmt.Sprintf("%.0f%%", *f*100)
+		},
 	}
 
 	return template.New("").Funcs(funcMap).ParseFS(templateFS,