refactor(logger): use standard log writer and flags

Replace direct os.Stderr usage with log.Writer() to maintain consistency
with Go's standard logging patterns. Add log.LstdFlags to include
timestamps in log output, which improves log readability and debugging.

Author: nametake

logger.go

@@ -3,7 +3,6 @@ package main
 import (
 	"encoding/json"
 	"log"
-	"os"
 )
 
 var _ logger = (*stdLogger)(nil)
@@ -21,7 +20,7 @@ type stdLogger struct {
 func newStdLogger(debug bool) *stdLogger {
 	return &stdLogger{
 		debug:  debug,
-		stderr: log.New(os.Stderr, "", 0),
+		stderr: log.New(log.Writer(), "", log.LstdFlags),
 	}
 }