Add DriftJudge AI role with verdict parsing

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

internal/ai/verdict.go

@@ -0,0 +1,47 @@
+package ai
+
+import (
+	"encoding/json"
+	"errors"
+	"fmt"
+	"strings"
+)
+
+// Verdict is the drift judge's call on a single window.
+type Verdict struct {
+	OnTask bool
+	Reason string
+}
+
+// ErrInvalidVerdict marks a parseable-but-unusable judge response.
+var ErrInvalidVerdict = errors.New("ai: invalid verdict")
+
+type rawVerdict struct {
+	OnTask bool   `json:"on_task"`
+	Reason string `json:"reason"`
+}
+
+// parseVerdict extracts and validates a Verdict from raw CLI output. It reuses
+// extractJSON and the shared empty/no-JSON sentinels. A drifting verdict with no
+// reason is unusable for the UI and is rejected as ErrInvalidVerdict.
+func parseVerdict(s string) (Verdict, error) {
+	if strings.TrimSpace(s) == "" {
+		return Verdict{}, ErrEmptyResponse
+	}
+	if strings.IndexByte(s, '{') < 0 {
+		return Verdict{}, ErrNoJSON
+	}
+	jsonStr, err := extractJSON(s)
+	if err != nil {
+		return Verdict{}, fmt.Errorf("%w: %v", ErrInvalidVerdict, err)
+	}
+	var raw rawVerdict
+	if err := json.Unmarshal([]byte(jsonStr), &raw); err != nil {
+		return Verdict{}, fmt.Errorf("%w: %v", ErrInvalidVerdict, err)
+	}
+	v := Verdict{OnTask: raw.OnTask, Reason: strings.TrimSpace(raw.Reason)}
+	if !v.OnTask && v.Reason == "" {
+		return Verdict{}, ErrInvalidVerdict
+	}
+	return v, nil
+}