Add DriftJudge AI role with verdict parsing

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

internal/ai/coach.go

@@ -42,3 +42,33 @@ Rules:
 
 User intent: ` + intent
 }
+
+// DriftJudge decides whether the current window is on-task for a commitment.
+// It takes primitives, not domain/evidence types, so ai stays a leaf package.
+type DriftJudge interface {
+	JudgeDrift(ctx context.Context, commitment, windowClass, windowTitle string) (Verdict, error)
+}
+
+// JudgeDrift makes Service satisfy DriftJudge over the same backend as Coach.
+func (s *Service) JudgeDrift(ctx context.Context, commitment, windowClass, windowTitle string) (Verdict, error) {
+	out, err := s.backend.Run(ctx, buildDriftPrompt(commitment, windowClass, windowTitle))
+	if err != nil {
+		return Verdict{}, err
+	}
+	return parseVerdict(out)
+}
+
+func buildDriftPrompt(commitment, class, title string) string {
+	return `You are a focus monitor. The user committed to a task. Decide whether their CURRENT window is on-task or a distraction.
+
+Respond with ONLY a JSON object, no prose and no code fences, exactly this shape:
+{"on_task": <true or false>, "reason": "<short explanation, one sentence>"}
+
+Rules:
+- on_task: true if the window plausibly serves the commitment, false if it is a distraction.
+- reason: one short sentence. REQUIRED when on_task is false.
+
+Commitment: ` + commitment + `
+Current window class: ` + class + `
+Current window title: ` + title
+}