Reject ambiguous configuration and reference bindings

internal/core/config/file_config.go

@@ -3,6 +3,7 @@ package config
 import (
 	"bytes"
 	"fmt"
+	"io"
 	"os"
 	"path/filepath"
 	"sort"
@@ -349,6 +350,12 @@ func ParseFileConfigYAML(data []byte) (FileConfig, error) {
 	if err := decoder.Decode(&fileCfg); err != nil {
 		return FileConfig{}, fmt.Errorf("decode yaml: %w", err)
 	}
+	var trailing any
+	if err := decoder.Decode(&trailing); err == nil {
+		return FileConfig{}, fmt.Errorf("config must contain exactly one YAML document")
+	} else if err != io.EOF {
+		return FileConfig{}, fmt.Errorf("decode trailing yaml document: %w", err)
+	}
 	return fileCfg, nil
 }