feat: add json log format option

Add log_format config field ("text" default, "json" for structured
JSON output) to support machine-readable logging.

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

cmd/oubliette/main.go

@@ -34,7 +34,14 @@ func main() {
 		level.Set(slog.LevelInfo)
 	}
 
-	logger := slog.New(slog.NewTextHandler(os.Stderr, &slog.HandlerOptions{Level: level}))
+	var handler slog.Handler
+	opts := &slog.HandlerOptions{Level: level}
+	if cfg.LogFormat == "json" {
+		handler = slog.NewJSONHandler(os.Stderr, opts)
+	} else {
+		handler = slog.NewTextHandler(os.Stderr, opts)
+	}
+	logger := slog.New(handler)
 	slog.SetDefault(logger)
 
 	ctx, cancel := signal.NotifyContext(context.Background(), syscall.SIGINT, syscall.SIGTERM)